Fourteen bovine ovaries of early pregnancy and nineteen of late pregnancy were perfused with citrated bovine blood. During perfusion, the venous blood was collected from each ovary before and after addition of lh and was analysed for progesterone content. Average progesterone synthesis rates were 43·37±1·12 and 4·57±0·99 μg/min before addition of lh, for ovaries of early and late pregnancy respectively, and increased to 7·43±1·98 and 7·72±1·33 μg/min, respectively, after addition of lh. There was no significant difference in the rates of progesterone synthesis between ovaries of early and late pregnancy at the 5% level and their response to lh was highly significant (P<0·01). These data indicate that a decrease in progesterone synthesis during late pregnancy in the bovine is not due to a reduction in the ability of the corpus luteum to respond to gonadotrophins.