Abstract: Mouse writhing response, hot-plate test and rat tail-flick tests were used to study the influence of reserpine and pargyline on the normal pain threshold in mice and rats and on the analgesia of morphine. The results are as follows: The writhing response in mice was inhibited after ip of reserpine at a dose of 2 mg/kg, or pargyline 100 mg/kg. The paw lick latency in mouse hot-plate test was prolonged following reserpine (1 mg/kg, ip) but not influenced, by pargyline. The tail-flick latency in rats was affected by neither reserpine (6 mg/kg, ip) nor pargyline (75 mg/kg,ip). The analgesic effect of morphine was antagonized by reserpine (0.5 mg/kg and 1 mg/kg ip in mice or 2 mg/kg ip in rats), and potentiated by pargyline (35 mg/kg ip in mice, or 50 mg/kg ip in rats). The antagonistic effect of reserpine against morphine was reversed by pargyline, and this effect was found to be dependent on the sequence of the administration of reserpine and pargyline. The analgesic effect of morphine was potentiated by administering pargyline and reserpine at the same time or by giving pargyline half an hour before reserpine. But the analgesic effect of morphine was decreased when reserpine was given before pargyline.