Following mating, intact follicles obtained from rabbit ovaries were utilized in a study of follicular steroidogenesis. Follicles were dissected from estrus and postcoital rabbit ovaries and incubated with and without LH in the presence of acetate–l–14C. Slices of rabbit corpora lutea were also incubated. Fats were extracted from the incubation mixtures with ether and then partitioned into polar and nonpolar fractions; the polar lipids were separated chromatographically to yield steroids for purification and recrystallization. The results of these studies extend the previously reported preovulatory rise and fall in steroidogenesis in whole rabbit ovaries to reveal the same pattern in in vitro steroid synthesis by isolated ovarian follicles. Stimulation of incorporation of radioactivity by LH occurred only in follicles from estrus rabbits. Follicular steroidogenesis was found to change qualitatively as well as quantitatively in the preovulatory interval. Relative to total 14C incorporation, follicles from estrus rabbits synthesized predominantly estrogens and testosterone. Follicles obtained 2 hr after coital stimulation made predominantly 17OH progesterone, while ovulatory follicles and corpora lutea synthesized progesterone. (Endocrinology92: 788, 1973)