In the ovaries of 75 guinea pigs from 42 days of gestational age to adult animals follicles of every developmental stage were investigated by means of histochemistry for the appearance of acid phosphatase, beta-glucuronidase and non-specific esterase in oocytes and follicle epithelial cells. For morphological control semi-thin sections were examined. Already in primordial follicles, oocytes exhibit a beta-glucuronidase and acid phosphatase reaction. Non-specific esterase can be detected in the oocytes and granulosa cells of secondary follicles. The three of them increase in activity during the growth of the follicles, but show no differences in the ontogenesis of the guinea pig. In the follicle epithelium the reaction of non-specific esterase is especially apparent in the cytoplasm of the cells surrounding the so called "Call-Exner-Bodies". The highest activity is seen in the content of these formations whereas the liquor folliculi does not show a considerable activity. Atresia of follicles can be detected both by enzyme histochemistry and visually in semi-thin sections, at least in advanced stages. The boundaries with intact follicles can at first be recognized by comparison of the follicle epithelium.