Intravenous administration of 0.04-0.08 mg/kg morphine sulfate reduced both sensory intensity and unpleasantness visual analogue scale (VAS) responses to graded 5 sec nociceptive temperature stimuli (45-51 degrees C) in a dose-dependent manner. The lower doses of morphine (0.04 and 0.06 mg/kg) resulted in statistically reliable reductions in affective but not sensory intensity VAS responses, possibly reflecting supraspinal effects on brain regions involved in affect and motivation. However, the highest dose of morphine tested (0.08 mg/kg) reduced both sensory and affective VAS responses to graded nociceptive stimuli as well as VAS sensory responses to first and second pain evoked by brief heat pulses. Morphine also had an especially potent inhibitory effect on temporal summation of second pain that is known to occur when intense nociceptive stimuli occur at rates greater than 0.3/sec. The results support current hypotheses about neural mechanisms of narcotic analgesia and further clarify the relative effects of morphine on sensory and affective dimensions of experimental pain. The derived morphine dose-analgesic response functions also provide a reference standard for quantitatively comparing magnitudes of different CNS-mediated forms of analgesia.