The effects of forskolin, which is an adenylate-cyclase activator, on the progress into meiotic prophase I was investigated in fetal rabbit ovaries of 26 days of gestation, using organ culture (stainless grid-lens paper method, according to Fell and Weiss, 1965) for 12 days. The explanted ovaries became flattened in their macroscopic shape, and necrosis in the deeper parts of the ovarian tissue was recognized in histology, caused by adhesion to the lens-paper, with the extra-ovarian connective tissue proliferating into the basic medium. In order to estimate the degree of progress into meiotic prophase I, the percentages of oogonia (mitotic and resting stage) and oocytes (leptotene, zygotene, pachytene and diplotene) were compared for all germ cells counted in 3 groups of organ cultures-0 μM (control), 20 μM (f20) and 100 μM (f100) forskolin. Although oocytes in diplotene were never observed, germ cells transforming into oocytes were recognized in all explanted ovaries of each group, except for the control, in which no oocytes in pachytene were found. According to the comparisons of the ratio of total oocytes to all germ cells counted in control (4.0%), f20 group (21.1%) and f100 group (9.0%), the degree of progress of meiotic prophase I in f100 group was not significantly different from that in control, although the degree of progress was somewhat higher. On the other hand, the degree of progress in f20 group was significantly higher than in control (p <0.01). It is suggested that the addition of forskolin to the basic medium of organ culture is effective in promoting progress into meiotic prophase I.