Mechanisms of hypnotic analgesia were investigated by examining changes in the R-III, a nociceptive spinal reflex, during hypnotic reduction of pain sensation and unpleasantness. The R-III was measured in 15 healthy volunteers who gave VAS-sensory and VAS-affective ratings of an electrical stimulus during conditions of resting wakefulness, suggestions for hypnotic analgesia, and attempted suppression of the reflex during non-hypnotic conditions. The H-reflex was also measured to monitor and control for general changes in alpha-motoneuron excitability. Hypnotic sensory analgesia was related to reduction in the R-III after controlling for changes in the H-reflex (R2 = 0.51, P < 0.003), suggesting that hypnotic sensory analgesia is at least in part mediated by descending antinociceptive mechanisms that exert control at spinal levels in response to hypnotic suggestion. The relationship between hypnotic affective analgesia and reduction in R-III approached significance (R2 = 0.26; P = 0.053). Reduction in R-III was 67% as great and accounted for 51% of the variance in reduction of pain sensation. In turn, reduction in pain sensation was 75% as great and accounted for 77% of the variance in reduction of unpleasantness. The results suggest that 3 general mechanisms may be involved in hypnotic analgesia. The first, implicated by reductions in R-III, is related to spinal cord antinociceptive mechanisms. The second, implicated by reductions in pain sensation over and beyond reductions in R-III, may be related to brain mechanisms that serve to prevent awareness of pain once nociception has reached higher centers, as suggested by Hilgard.(ABSTRACT TRUNCATED AT 250 WORDS)