||The aim of this study was to determine whether males have appreciable individual differences in the hydraulic conductivity (Lp) and permeability coefficient for glycerol (Ps) of the sperm cell membrane. The permeability parameters were determined in semen of seven AI bulls of Holland Genetics (The Netherlands), four ejaculates per bull, at 22, 10, and 0 °C. The stopped-flow technique was applied in order to capture rapid cell volume changes under anisosmotic conditions in the absence or presence of glycerol. This technique utilizes a concentrationdependent self-quenching entrapped fluorophore. The resulting cell volume changes were used in three-parameter fitting calculations to compute Lp in the absence of permeant solutes and Lp and Ps in the presence of the permeating solute glycerol. Averaged over all (7) bulls, Lp in the absence of glycerol was 0.277 ± 0.037, 0.152 ± 0.017, and 0.099 ± 0.007 lm/min/atm (mean ± SD) at 22, 10, and 0 °C, respectively, yielding an Arrhenius activation energy (Ea) of 7.45 kcal/mol. In the presence of 0.5 M glycerol Lp, Ps, and the reflection coefficient (r) at 22 °C were 0.074 ± 0.015 lm/min/atm, 0.825 ± 0.333 · 10-3 cm/min, and 0.92 ± 0.103 (mean ± SD), respectively. Bulls varied in the values of Lp (in presence of glycerol) and Ps measured at 22 °C. There was no appreciable variation among bulls in the Ea value. Lp in the presence of glycerol varied among bulls from 0.056 ± 0.007 to 0.098 ± 0.045 lm/min/atm (mean per bull ± SD). Ps varied among bulls from 0.479 ± 0.096 to 1.34 ± 0.521 · 10)3 cm/min. However, there was also considerable variation in the Lp and Ps values between ejaculates within a bull. There were no significant correlations between Lp and Ps, between Lp and the non-return rate (NRR), between Ps and NRR, between Lp and the motility of the fresh semen, or between Ps and the motility. Glycerol had a significant effect on Lp. The average Lp value obtained in the presence of glycerol at 22 °C, was 3.7 times lower than Lp obtained in the absence of glycerol. This is attributed to the ‘‘dragging’’ effect, that is, the glycerol molecules hamper the movement of water through the membrane. The variation among bulls in Lp and Ps was only limited, the highest and lowest values differing by a factor of 1.45 and 2.8, respectively. Still, this indicates that individual males could require individual adjustments of the cooling protocol.