Administration of the carcinogenic hydrocarbon 9:10-dimethyl-1:2-benzanthracene (DMBA) in mice causes destruction of the oocytes in the ovaries resulting in a premature depletion of the germ cells. Subsequently ovarian tumours develop. It was investigated to what extent fertility is influenced during the process of oocyte disappearance in immature or mature mice treated with DMBA. The first parturition was not delayed and as long as the treated mice remained fertile, litters were born at normal intervals. However, the fertility span was considerably shortened due to the premature depletion of oocytes, but in contrast to the control mice, young were born to the DMBA-treated mice until no or very few oocytes remained. As a consequence of the shortened fertility span, the experimental mice gave birth to fewer litters than the control mice. The litter sizes were also smaller; the possible reason for this reduction in size is discussed in relation to the DMBA-induced reduction in the pool of small oocytes. One year after DMBA-ingestion a number of mice had developed ovarian tumours. Using the time of birth of the last litter as an indicator of the time at which the ovaries were depleted of oocytes it is shown that the earlier the germ cells disappeared, the earlier tumours developed.