||Previous studies indicated that genetic influences (e.g. sire of calf and breed of calf) as well as environmental influences (e.g. month of calving) are related to characters like birth weight, gestation length, ease of calving, stillbirth frequency and congenital defects. This research project was initiated to investigate the contribution of the circulating oestrogens in pregnant cows to some of these relationships. The above mentioned relationships could be accomplished via the hormonal mechanism. Variations in the parturition characteristics, therefore, might be related to quantitative variations in the hormonal equilibrium. The initiation of parturition is very much dependant on the calf's genotype, therefore this study concerns the oestrogenic hormones, for these hormones are predominantly produced by the foeto-placental unit, during gestation. In the pregnant cow progesterone seems to be produced only by maternal organs. No direct relationship with gestation is known for other steroids while peptide hormones are mainly produced by the maternal pituitary. The function of peptide hormones and of corticosteroids in the process of parturition has not been described in this study. From the available analytical methods, which were developed for the oestrogen analysis in human urine, methods were developed for the quantitative analysis of oestradiol-17αand oestrone in urine as well as faeces of pregnant cows. The urinary oestrogen excretion rate is expressed as the oestrogen/creatinine ratio (μg/g), because it is difficult to collect 24 hour urine samples of many experimental animals. The ratio between the oestrogen concentration and the creatinine concentration proved to be a little higher at night as a result of a significant circadian rhythm in the creatinine output.In order to study the applicability of the analytical method and the total variation in the urinary oestrogen excretion rate, within cows and between cows, two preliminary experiments were carried out during 1966, 1967 and 1968. In total 168 cows, from 4 experimental farms, were sampled regularly, 60 of these cows participated in both experiments during 2 consecutive pregnancies. In order to study the genetic influences on the oestrogen excretion rate, the material of experiment I included 21 and experiment II 41 sets of identical twins.All oestrogen/creatinine ratios had to be transformed to a square root, because the raw data deviated significantly from a normal distribution.Analysis of variance showed that the oestrogen/creatinine ratios as well as their standard deviations increased with the stage of gestation. Between the mean oestrogen excretion rates of cows, grouped together according to their herds, months of calving, birth weight of calves and breed crosses or pure breeds, differences were found within the stages of pregnancy.High heritability estimates were calculated from the data obtained from identical twins in the second experiment. High correlation coefficients could be calculated between the oestrogen excretion rates within cows of two consecutive pregnancies.In 10 normal and 11 induced parturitions (with flumethasone) the variation in the urinary oestrogen excretion rate was studied immediately before, during and after parturition. The urinary oestradiol-17αexcretion rate increases up to the last day before normal parturition and decreases within a few hours before parturition. Within two days after normal - as well as induced - parturition the urinary oestrogen excretion rate drops to such a low level that it cannot be estimated accurately enough by the applied analytical method. In induced parturitions a dramatic increase in the oestrogen excretion rate occurs between 12 hours after medication and the moment of parturition.In combination with balance-trials the ratio between oestrogens excreted in the faeces and in the urine was found to vary within animals from week to week and between animals. During 8 weeks 32 daily collections of urine and faeces from 4 cows were analysed. This experiment showed that at least 25 of the total oestrogens is excreted via the faecal route.In a specially designed experiment in one a.i.-centre, 5 groups of 25 cows each group pregnant to a bull, selected for producing a high or low incidence of stillbirths in heifers, showed a relationship between stillbirth frequency and urinary oestrogen excretion rate during pregnancy. Cows bred to bulls producing a high stillbirth frequency in heifers showed a low urinary oestrogen excretion rate at 260 days. It was concluded that a difficult calving, in which parturition proceeds slowly and lasts long, might be caused by a coincidence of a high birth weight of the calf and a low oestrogen excretion rate by the dam.Friesian cows, bred to a Charolais bull, frequently show a difficult parturition coinciding with a heavy calf. Therefore, in such cows a low urinary oestrogen excretion was anticipated.One animal of each pair of 6 identical Friesian twins was bred to a Charolais bull and the other was bred to a Friesian bull. The excretion of oestradiol-17αwas significantly lower in the cows bred to the Charolais bulls during the last three weeks of pregnancy than in the cows bred to the Friesian bulls. The excretion of oestrone was significantly lower in the cows bred to the Charolais bulls at 260 days after conception when compared to their twin sisters bred to Friesian bulls.