Temporal changes in follicular oestradiol production induced in vitro and in vivo by LH were studied. In-vitro changes were measured by incubating preovulatory rat follicles for 12 h, changing the medium every 2 h. Follicles isolated at various intervals after an injection of 10 i.u. hCG were incubated for 2 h to measure changes in oestradiol production in vivo. In both studies there was an increase in oestradiol production lasting 4 h followed by a sharp decline. Progesterone production was also increased by LH in vitro or hCG in vivo, but remained high. A second exposure to LH did not raise oestradiol synthesis, but increased progesterone synthesis in vitro only. The decline in oestradiol production is most probably due to a decrease in C17-20 lyase activity, because addition of testosterone, but not of 17 alpha-hydroxyprogesterone, increased oestradiol production. Incubation of preovulatory follicles in the absence of LH or incubation of follicles derived from animals in which the spontaneous LH surge was blocked by an injection of pentobarbitone sodium also resulted in a decrease of oestradiol and an increase in progesterone production. This oestrogen-progesterone shift was also caused by a decrease in C17-20 lyase activity. The results demonstrate that the changes in steroid production in vivo and in vitro are similar and occur in the presence and absence of LH. It is concluded that the decrease in oestradiol production is dependent on the decrease in the activity of enzymes converting progesterone to aromatizable androgens.