A severe negative energy balance during first lactation may result in poor reproductive performance in the second litter. Allowing the sow recovery time after weaning by inseminating the sow the second estrus after weaning (skip a heat) improves reproductive performance. Postponing estrus for a shorter period after weaning using daily altrenogest administration has also been found to influence reproductive performance. The aim of this thesis was to develop a better understanding of consequences of altrenogest after weaning for follicle development and subsequent reproductive performance. Therefore, a first experiment investigated follicle development during and after post weaning altrenogest treatments and related this to subsequent fertility. It showed that follicle size increased during altrenogest treatment (independent of dose and duration), but no effects were found on fertility parameters on day 5 of gestation. Therefore, a second experiment studiedthe effect of duration of treatment on follicle development and subsequent farrowing rate and litter size. This experiment showed again an increase in follicle size and also showed that long altrenogest treatments (15 d) improve fertility, but that shorter treatments (8 d) reduce farrowing rates in sows with large follicles at weaning. So, to prevent outgrowth of follicles before weaning a third experiment started altrenogest treatment 3 d before weaning. This did not suppress follicle growth, but fertility was improved after altrenogest treatment in primiparous sows with compromised body condition at weaning. A fourth experiment attempted to stimulate follicle growth before weaning using split-weaning (reducing litter size to the 6 smallest piglets 3 d before complete weaning) and found that split-weaning resulted in lower embryonic survival, especially in sows with large follicles at weaning or high follicle growth during treatment. Because in all experiments sows showed follicle growth during altrenogest treatment, it was suspected that LH release was not completely suppressed during altrenogest treatment. Therefore, a fifth experiment investigated LH pulsatility during the last day of altrenogest treatment and indeed showed that LH release was suppressed during only a part of the 24 h between altrenogest administrations. Finally a last experiment showed a release pattern of both FSH and estradiol that varied over the day related with the moment of daily altrenogest administration. Further, a decrease of estrogenic activity was found during the second week of post weaning altrenogest treatment, probably as a result of reduced LH responsiveness. The level of estrogenic activity was related to weight loss during lactation. Therefore, it is assumed that LH and FSH release during altrenogest treatment stimulate follicle growth, but that levels are not high enough to sustain outgrowth of the follicles to pre-ovulatory sizes and, as a result, follicles go into atresia after on average 5-8 d of treatment. This may explain why long altrenogest treatments (12-15 d) result in improved fertility, short altrenogest treatments (3-4 d) have little effect on fertility and intermediate altrenogest treatments (5-8 d) may reduce fertility. As there is large variation between sows (parity, lactational burden, follicle size), this may also affect their response to altrenogest treatment. In general, to improve reproductive performance, it is recommended to start altrenogest treatment 3-6 h before weaning and to apply intermediate treatments (5-8 d) only in primiparous sows that suffered a severe lactational burden and are in low body condition at weaning.