The reinforcing and subjective effects of fentanyl, an opioid analgesic, were tested in ten healthy volunteers without histories of drug abuse, as a function of the temperature of a water bath in which the volunteers' forearms were immersed. The temperatures were body-temperature (37°C), moderately cold (10°C), and very cold (2°C). A discrete-trial choice procedure was used in which, in each session, volunteers sampled 50 μg of fentanyl (delivered as a bolus via an infusion pump) and saline, and then on three successive trials, chose between the two. Volunteers then had to immerse their non-dominant forearm in the water bath 5 min after a drug delivery. Fentanyl was chosen on 77% of choice occasions in the 10°C and 2°C water conditions, which was significantly different from chance levels, and on 60% of choice occasions in the 37°C water condition, which did not differ from chance levels. Several subjective effects of fentanyl were also modulated by the temperature of the water bath. We conclude that in the context of a painful stimulus, 50 μg of fentanyl functions as a reinforcer in non-drug abusers.