Carcinomas of the rat prostate induced by a single injection of N-methyl-N-nitrosourea, 7,12-dimethylbenz(a)anthracene, and 3,2'-dimethyl-4-aminobiphenyl, after sequential treatment with cyproterone acetate and testosterone propionate, were evaluated as potential animal model for prostatic cancer. All ten carcinomas examined were located in the dorsolateral prostate region and did not involve the distal parts of the seminal vesicle and coagulating glands. The incidence of urinary obstruction leading to the animals' death was 6 of 10 rats, and metastases in the lung, abdominal lymph nodes, and/or liver also occurred in 6 of 10 rats. The tumors were invasive adenocarcinomas, showing frequent perineural invasion and a variable degree of differentiation. There were ultrastructural similarities with human prostatic carcinoas, such as intracellular lumina. Plama acid phosphatase was increased. Enzyme histochemical analysis revealed similarities with the Dunning R3327H and -HI prostatic carcinomas but was not helpful in determining the site or origin of the tumors. The gross and microscopic appearance of the tumors and the observation of preneoplastic lesions exclusively located in the dorsolateral prostate suggest this lobe as site origin of the carcinomas. Preneoplastic lesions (n = 9) included atypical hyperplasias (n = 5) and lesions with all histological characteristics of carcinoma except for local invasion and metastases, which were classified as carcinoma in situ (n = 4). Although androgen sensitivity could not be assessed, the observed characteristics of the tumors [their long latency time (46-80 weeks), the presence of preneoplastic lesions, and the short duration of the treatment, leaving the animals intact] all indicate that the present approach is a valid animal model for the study of prostatic carcinogenesis. Chemicals/CAS: cyproterone acetate, 427-51-0; testosterone propionate, 57-85-2; 2',3-dimethyl-4-aminobiphenyl, 13394-86-0; 9,10-Dimethyl-1,2-benzanthracene, 57-97-6; Acid Phosphatase, EC 3.1.3.2; Aminobiphenyl Compounds; Methylnitrosourea, 684-93-5