Alloxan diabetes caused a marked stimulation of endogenous oxygen uptake by rat testis tissues. However, the oxygen uptake in the presence of added glucose or pyruvate as substrate was significantly reduced. The ability of the testis tissues to utilize these substrates and to produce lactate was significantly inhibited in the diabetic animals. Alloxan diabetes also increased the levels of cholesterol, cholesterol esters, non-esterified fatty acids, triglycerides and phospholipids in rat testis tissues. Treatment of the diabetic rats with ascorbic acid caused more or less a normalization of all the parameters tested.