Effect of oral administration of crude aqueous neem extract on serum testosterone and other blood constituents was studied in the male Wistar rats for 10 weeks. The neem treatment resulted in significant decreases (p < 0.01) in total testosterone, total bilirubin and K+ in serum. There were also increases (p < 0.05) in packed cell volume, mean corpuscular haemoglobin concentration, red blood cell, white blood cell and lymphocyte counts without showing any cytotoxic effects in the body.