Testosterone (T) levels in the plasma of male laboratory rats and mice were measured by radioimmunoassay. There was a striking individual variation with values ranging from less than 1 ng/ml to over 30 ng/ml in mice of the same age and strain housed under identical conditions. Using chronic indwelling catheters inserted into a jugular vein, blood was collected from adult conscious male rats every 24 hr for 4 or 8 days and every 30 min for 2l/2 or 8 hr. Considerable differences in plasma T levels were observed between different animals, and 2– to 5–fold fluctuations of T concentrations in the plasma were detected between samples collected from the same animal at different times. These large and irregular changes in plasma levels of T are unlike the fairly stable levels observed in the human but bear certain resemblance to the pulsatile release of T described in bulls and rams and perhaps also to the social dominance related differences in plasma T levels in Rhesus monkeys. (Endocrinology92: 1223, 1973)