The prostates of 1775 (614 control and 1161 experimental) 2-year-old F344 rats from 12 different carcinogen bioassays conducted by the Bioassay Program of the NCI and the NTP were evaluated histologically. The incidence of prostatic lesions including (atypical) hyperplastic foci, adenomas, and carcinomas was 6.8%. There was no difference in the type or incidence of the lesions between treated and untreated animals. Adenomas or carcinomas were found in 71 (4.0%) of the rats, primarily as incidental findings. The number of tumors and hyperplastic foci varied from laboratory to laboratory depending on the anatomical localization of the plane of the section. Most of the neoplasms were found in the ventrolateral lobes of the prostate (ventral prostate). When adequate sections were prepared of the ventral lobe, 10-20% of the prostates had these proliferative lesions. The lesions were usually small and originated in the epithelium of the alveoli and small ducts and were usually small and originated in the epithelium of the alveoli and small ducts and were not associated with the common inflammatory lesions of the rat prostate. Thin fibrous capsules were formed in a few of the larger tumors. Metastases were not observed but there was local invasion into alveoli, ducts and interstitial connective tissue. Evidence is presented that the atypical hyperplasias progress to adenoma and carcinoma. The F344 rat offers a potential model for the study of latent prostatic preneoplastic and neoplastic lesions.