Gonadal functions, with special reference to blood levels of sex-related markers such as 17beta-estradiol (E2), free testosterone (free Te) and lutenizing hormone (LH), were examined in male OLETF (Otsuka Long Evans Tokushima Fatty) diabetic rats, a model of human type 2 diabetes mellitus. Male rats of the OLETF strain and male rats of the LETO strain, which act as a control of OLETF, both supplied by Otsuka Pharmaceutical Co., Ltd. (Tokushima, Japan), were periodically examined for blood levels of E2, free Te and LH at the age of 4, 5, 32, 40 and 64 weeks. The weight of the testis, the number of sperm contained within and histological findings of the testis were comparatively studied in both strains. Glucose and insulin (IRI) at fasting were examined to evaluate the homeostasis model assessment (HOMA) index. In order to investigate any sex hormone imbalance, sex hormone-binding globulin (SHBG) was measured by a dextran- charcoal assay. All of the OLETF rats became diabetic at the age of 32 weeks. There were no significant differences between OLETF and LETO rats regarding free Te, E2, LH or SHBG during the observation period from 4 to 64 weeks. Testis weight was significantly decreased in OLETF rats at 32 and 64 weeks and sperm counts at 64 weeks of age were also significantly decreased. Histologically, there was seminiferous tubule atrophy in the OLETF rats at 64 weeks of age. A significant negative correlation between testis weight and fasting blood glucose, as well as HOMA index, was observed in OLETF rats. In male diabetic OLETF rats, with a variety of hypogonadisms such as atrophy of the testis and low sperm count, the serum levels of E2, free Te, LH and SHBG were normally preserved.