Prostatic adenocarcinomas were induced in 5 out of 20 Wistar rats upon a single administration of 50 mg/kg N-nitroso-N-methylurea (NMU). The rats were pretreated with a daily dose of 50 mg/kg cyproterone acetate for 3 weeks followed by 3 daily injections of 100 mg/kg testosterone. All tumours developed in the dorsolateral prostate and were invasively growing. In 2 cases distant metastases were found. Three proliferative lesions classified as carcinomas in situ were also found in the dorsolateral prostate. A total of 7/20 animals (35%) carried an adenocarcinoma and/or a carcinoma in situ. In addition, 6 epithelial hyperplasias were observed in the dorsolateral and 1 in the ventral prostate of non-tumour-bearing rats. The method described may provide a good animal model for cancer of the prostate and lead to a better understanding of prostatic carcinogenesis.