Pain threshold for thermal stimulus after morphine, naloxone alone and naloxone in combination with morphine was studied in male rats before and after three days treatment with testosterone. It was also determined 15 days after gonadectomy and administration of testosterone in such rats. There was significant reduction in morphine analgesia after administration of testosterone and also after gonadectomy. Naloxone increased the pain threshold in gonadectomised rats and it enhanced morphine induced analgesia instead of antagonising it. Naloxone, however, had no effect on morphine analgesia in testosterone treated control rats and gonadectomised rats.