Rhesus monkeys were restrained in chairs from which their tails hung free so that their tails could be immersed into a thermos of water. Monkeys consistently kept their tails in 38-40 degrees C water for at least 20 sec, but withdrew them from 55 degrees C water in 1-4 sec. Tail withdrawal latencies from 55 degrees C water remained consistent over a period of 3 hr. Morphine produced dose-dependent increases in tail withdrawal latencies from 55 degrees C water, whereas pentobarbital, haloperidol, and phencyclidine did not increase tail withdrawal latencies except at doses that produced marked sedation.