PURPOSE: The mechanism of erectile dysfunction in type II non-insulin dependent diabetes mellitus (NIDDM) has not been well demonstrated. The aims of this study were to investigate erectile function and cavernosal TGF-beta expression in Otsuka Long-Evans Tokushima Fatty (OLETF) rats, which develop NIDDM naturally. MATERIALS AND METHODS: Twenty male OLETF rats and ten control LETO rats were included in this study. After 30 weeks, blood glucose levels and body weights were measured. Systemic arterial pressure and intracavernosal pressure were measured before and after pelvic nerve stimulation. Cross sections of the penis were processed for histologic and immunohistochemistry examinations. RESULTS: The mean body weights and mean blood glucose levels were significantly higher in the OLETF group (645.0 40.0 g; 130.8 55.6 mg/ml) compared to control group (538.0 14.4 g; 84.1 14.7 mg/ml), respectively (p<0.05). Nerve stimulation-induced peak intracavernosal pressure (mmHg) was significantly decreased in the OLETF group (41.3 15.4 mmHg) compared to control group (83.3 15.8 mmHg) (p<0.05). In the OLETF rats, the collagen connective tissue in the corpus cavernosum showed a dense and irregular arrangement. The immunoreactivity for TGF-beta 1 was prominent in collagen fibers, fibroblast and smooth muscle in the OLETF rats. CONCLUSIONS: In this study, OLETF rats showed significantly decreased erectile function compared with control rats, and histologic examination revealed cavernosal fibrosis and increased TGF-beta 1 expression. These results imply that the OLETF rat may be a good model to study the mechanism of erectile dysfunction in NIDDM.