An aromatase inhibitor was identified in human follicular fluid. We sought to examine the effects of this inhibitor on granulosa cells, theca cells, a mixture of granulosa and theca cells, and follicles. Granulosa cells, theca cells, the mixture of granulosa and theca cells, and follicles were incubated with [3H]-thymidine and aromatase inhibitor at 37°C for 40 h in Ham F-10 medium. The radioactivities of acid-insoluble fractions were counted. Aromatase inhibitor markedly inhibited the DNA synthesis of granulosa cells and follicles, but it did not inhibit the mixture of granulosa and theca cells. This suggested that aromatase inhibitor was a factor in the follicle paracrine system and acted mainly on granulosa cells