In adult wild mallards the penis weighs about 0·6 gms (0·05 per cent of body weight) for most of the year but increases to a weight of almost 3 gms. (0·3 per cent of body weight) at the height of the breeding season. In immature birds which had reached adult body weight the average penis weight was only 0·03 gms. (0·0026 per cent of body weight). The breeding season penile enlargement could be prevented by castration early in the spring. Evidence that Testosterone administration can cause enlargement of the penis in adult mallards was also obtained. Unilateral adrenalectomy did not effect penile or testicular weight.