The porcine antral follicles, 4-6 mm in diameter, were dissected from the ovaries of mature pigs, and then granulosa and cumulus cells were isolated from each follicle. In atretic follicles, apoptotic cells were observed histochemically among granulosa cells only. In atretic follicles, high activity of neutral Ca²⁺/Mg²⁺-dependent endonuclease by which the chromatin DNA is degraded at internucleosomal sites in the cells undergoing apoptosis was noted only in granulosa cells but not in cumulus cells. Low activity of the Ca²⁺/Mg²⁺-dependent endonuclease was observed in both types of cells obtained from healthy follicles. Low activities of neutral Ca²⁺-dependent endonuclease, neutral Mg²⁺-dependent endonuclease and acidic cation-independent endonuclease were demonstrated in all cases. A good correlation between the Ca²⁺/Mg²⁺-dependent endonuclease activity of granulosa cells and the progesterone/estradiol ratio of follicular fluid in each follicle, which provides an index of follicular atresia in pig ovaries, was found. In vitro study showed that the specific inhibitors for interleukin-β converting enzyme (ICE) like cysteine proteases could stop the apoptotic process induced by specific monoclonal antibody to granulosa cell membrane protein in primary cultured granulosa cells. These results suggest that apoptosis occurs in granulosa cells but not cumulus cells in the atretic antral follicles in pigs, and ICE like protease (s) is involved in intracellular signal transmission of granulosa cell apoptosis of the atretic antral follicles in pigs.