In the present study, we examined the possibility that granulosa cell death during ovarian follicular atresia occurs by apoptosis (programmed cell death). To investigate this possibility, atresia was induced in immature female rats by injecting 15 IU PMSG. Controls received either vehicle or no treatment. PMSG-treated animals were killed on days 1-5 post-injection while controls were killed on days 1 or 5. The onset of atresia was assessed histologically by light microscopic inspection of 5 microns tissue sections and functionally by quantification of serum progesterone and estrogen levels. Apoptosis is characterized by the cleavage of genomic DNA into oligonucleosomal length fragments by a Ca2+/Mg(2+)-dependent endogenous endonuclease. Such fragments form a distinctive ladder pattern when separated electrophoretically. Accordingly, the occurrence of apoptosis in granulosa cells was assessed by examining the pattern of fragmented DNA in cell lysates after agarose gel electrophoresis. Gels were stained with ethidium bromide and DNA visualized by UV transillumination. The earliest morphological signs of atresia were detected 4 days after PMSG injection as evidenced by degeneration and detachment of granulosa cells from the basal lamina. Serum estrogen increased from basal to levels 7-fold over controls by day 3 after PMSG treatment, falling to control values by day 4 and thereafter. In contrast, progesterone remained basal for the first 3 days, rising to levels 3-fold and 8-fold above controls 4 and 5 days after PMSG treatment, respectively. Such shifts in the ratio of estrogen to progesterone production are known to be characteristic of follicular atresia. Finally, electrophoretic analysis of low mol wt DNA in granulosa cell lysates revealed a definitive ladder pattern of oligonucleosomal length DNA fragments (characteristic of apoptosis) on days 4 and 5 after PMSG injection. This pattern was not detectable on days 1 and 2 after treatment. Lysates obtained 3 days after PMSG treatment showed a faint apoptotic-like pattern of DNA fragments; a result consistent with other systems in which DNA cleavage begins before any morphological signs of death. Interestingly, a ladder pattern of DNA fragments was present in control lysates suggesting that granulosa cell death under normal (vs. induced) conditions of atresia in immature rats occurs by apoptosis. These data demonstrate an intimate association between apoptotic-like events and dying granulosa cells and thus support the possibility that apoptosis is involved in the induction of follicular atresia.