We examined the ability of systemic morphine to diminish the sensory discriminative features of noxious heating. Monkeys were trained to perform a thermal detection task, and the time until their detection of small increases in heating was used as a measure of the perceived intensity of pain. Relatively small doses of morphine sulfate (0.1, 0.3 and 1.0 mg/kg, i.m.) increased the time until detection of graded temperature increases (0.4-1.0 degrees C) from a noxious 46 degrees C baseline. These effects were generally dose-dependent, reversed by systemic naloxone, and did not result from changes in attentional, motivational or motoric aspects of the monkeys' behavior. Furthermore, the effects of morphine were more pronounced on detection of temperature shifts that were near threshold. These findings indicate that doses of morphine in the therapeutic dose range for humans alter the perceived intensity of noxious heat in monkeys.