The antitumour effect of CGS 16949A, an aromatase inhibitor, was investigated in rats with mammary tumours induced by 7,12-dimethylbenz[a]anthracene. A dose-dependent antitumour effect was observed after daily oral administration of CGS 16949A for 3 weeks. The tumour did not recur in the groups treated with 4.0 and 8.0 mg/kg per day. The complete remission rate increased and the time required to achieve complete remission became shorter with increasing daily doses. After daily administration for 3 weeks, a significant antitumour effect was observed in the group treated with CGS 16949A plus tamoxifen compared with that seen either with CGS 16949A or with tamoxifen alone. At the end of treatment, the group treated with CGS 16949A had significantly decreased oestradiol-17 beta and prolactin levels and increased levels of follicle stimulating hormone, but oestrone was not affected.