This report describes the variation of testosterone (T) levels in male Wistar rats (250-300 g) bled under typical laboratory conditions. Plasma T in 114 rats ranged from 2-48 nmol/l (median 13.1; 95% confidence range, 10-15) and varied significantly (P < 0.01) during the day (07.30-16.30 h) with an acrophase between 09.00 h and 13.00 h. The distribution of plasma T levels was skewed (P < 0.05) and could be divided into basal values < 26 nmol/l and peaks > 26 nmol/l. Testosterone in testicular tissue and testis veins also varied during the day and was lower (P < 0.02) between 14.30 h and 16.30 h than from 11.30-13.30 h. It was shown that plasma T may differ significantly (P < 0.02) between groups (n = 6-7) of untreated rats bled under identical conditions and it is suggested that gonadotrophin stimulation be included in experimental designs to reduce the difficulty of differentiating between normal variability and treatment effects.