The effect of a low-dose treatment (10 mg/day) of stanozolol on the anaerobic threshold (AT) and on maximal velocity (Vmax) was studied in ten well-trained runners in a 6-week trial experiment. The serum levels of testosterone (T), interstitial cell-stimulating hormone (ICSH), and of follicle-stimulating hormone (FSH) were determined before, during, and after the steroid administration. No improvements of AT and Vmax were found. No improvement of the competition running times was observed during the experimental period. The ICSH and FSH serum concentrations did not change significantly, while a marked decrease of the T serum concentration was observed. It is concluded that a low-dose treatment of stanozolol, sufficient to depress the testicular function, does not improve athletic performance.