Cebus albifrons monkeys received electrical stimulation of the hindlimbs over a wide range of intensities. On trials signalled by a blue light, the animals were permitted to escape shock by pressing a disc, or shock was terminated after 8 sec (free escape). Escape force (disc pressure) was found to increase as stimulation intensity increased well beyond escape threshold, while shock duration curves reached plateau at the mid-range of intensities. The shock duration curves generated by free escape responses should be comparable to pain detection functions obtained by similar operations in humans, and the curves were stable over months of testing, as is generally found in pain-detection studies. On trials signalled by a red light, the animals received intense tail shock immediately after escape responses (punished escape), or, if they endured leg shock for 8 sec without escaping, then they could avoid tail shock with a panel press. The shock duration curves generated by punished escape responses should be comparable to pain tolerance functions as defined for human subjects, and the escape thresholds were considerably higher on redlight trials. As in human studies, the tolerance curves were not stable over repeated testing sessions, and some feature of the paradigm forced a progression toward extremely high levels of tolerance.The behavioral assessment of pain in laboratory animals has progressed from the notation of a variety of reflex reactions following noxious stimulation (Carroll and Lim, 1960;Evans, 1961; Randall and Selitto, 1957;Siegmund, Cadmus, and Lu, 1957) to the measurement of aversion thresholds by operant techniques such as titration (Fields and Glusman, 1969;Weiss and Laties, 1961;Weitzman and Ross, 1962) to the realization that degree of aversiveness can be operationally defined along a continuum of responsiveness that extends over a wide range of stimulus intensities