The 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, high activity of neutral Ca2+/Mg 2+-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 Ca 2+/Mg2+-dependent endonuclease was observed in both types of cells obtained from healthy follicles. Low activities of neutral Ca 2+-dependent endonuclease, neutral Mg2+-dependent endonuclease and acidic cation-independent endonuclease were demonstrated in all cases. A good correlation (r=0.892) between the Ca2+/Mg 2+-dependent endonuclease activity of granulosa cells and the progesterone/estradiol-17β ratio of follicular fluid in each follicle, which provides an index of follicular atresia in pig ovaries, was found. These results suggest that the Ca2+/Mg2+-dependent endonuclease, and not the Ca2+-dependent endonuclease, the Mg2+- dependent endonuclease or the cation-independent endonuclease, is involved in granulosa cell apoptosis of the atretic antral follicles in pigs.