SUMMARY The induction of ovarian tumours in mice by 9,10-dimethyl-1,2-benzanthracene (DMBA) is preceded by the disappearance of oocytes and follicles. The effect of DMBA on the oocyte population was determined in immature and in mature mice by performing total and differential oocyte counts at different intervals after oral and intraperitoneal administrations of the carcinogen. In orally treated animals the number of small oocytes was quickly reduced followed by a decrease in the number of growing and large oocytes; in addition, in intraperitoneally treated animals, some growing and large oocytes were directly destroyed. The sensitivity of the germ cells to DMBA was independent of the age of the animals. In normal mice, as well as in immature mice treated with DMBA, the number of small oocytes determined the proportion which was stimulated to develop into growing and large oocytes.