Three experiments evaluated the reliability and sensitivity of an interactive multiple random staircase (MRS) assessment of painful thermocutaneous sensations. One hundred and sixteen subjects used a 4-point category scale (no pain, mild, moderate, intense) to rate the intensity of sensations produced by 3-sec thermal stimuli applied to the volar forearm at 20-sec intervals by a 1-cm diameter contact thermode. Each of the 3 intervals between the 4 verbal responses was defined as a boundary. A pair of staircases was associated with each boundary. On each trial, 1 of the 6 staircases was chosen randomly and the stimulus intensity indicated by that staircase presented. The response to that stimulus determined the intensity presented by that staircase the next time it was randomly selected. Responses above the associated boundary decreased stimulus intensity, responses below the associated boundary increased stimulus intensity. In the first experiment, 1 staircase from each of the 3 boundaries began at 43 degrees C and 1 began at 48 degrees C. Staircases for each boundary converged to within 0.3 degree C after 12 trials/staircase. The sensitivity of the method to a narcotic analgesic was assessed by open (exp. II) and double-blind (exp. III) intravenous infusion of 1.1 micrograms/kg fentanyl. Administration of fentanyl increased staircase temperatures, indicating that these higher temperatures were now required to elicit the same verbal responses. This shift in temperature reached a peak effect 11 min after fentanyl administration. These results suggest that this method provides a reliable measure of sensory magnitude in units of stimulus intensity. It does not require assumptions about psychological units of pain.(ABSTRACT TRUNCATED AT 250 WORDS)