Macaca nemestrina monkeys received unilateral interruption of the spinothalamic tract, producing contralateral hypalgesia and a bilateral decrease in amplitude of the flexion reflex. These effects on operant escape and reflex responses to electrocutaneous stimulation (ES) were monitored for months to evaluate relationships between the extent of each lesion and the presence or absence of recovery from the early postoperative deficits. Before surgery, the animals were trained to perform an operant response that terminated ES to the lateral calf of either leg. The durations of ES tolerated by each monkey were inversely related to stimulus intensities within the pain sensitivity range of human subjects. The vigor of operant escape responses and the frequency of intertrial pulls of the manipulandum were directly related to stimulus intensity. Following anterolateral chordotomy at an upper thoracic level, these measures revealed a contralateral hypalgesia for each animal. Operant responsivity to stimulation contralateral to the lesion recovered to control levels for 7 animals (group R). Sustained contralateral recovery of operant reactivity was not observed for 8 monkeys (group U). Most of the lesions in groups R and U were similar in extent and location, involving the classical distribution of the spinothalamic tract (in the anterolateral and ventral columns). Thus, recovery was not determined solely by lesion configuration. However, when recovery did occur, it was associated with medially extensive lesions. A subgroup of 3 unrecovered animals received superficial lesions that did not substantially involve the gray matter or the ventral columns. For all animals, reflex magnitudes were initially diminished bilaterally and then increased over months of testing. Reflex recovery was greater for the animals that demonstrated recovery of intentional reactions to nociception (group R). An ipsilateral hyperreflexia became apparent for group R. Contralateral recovery to normal levels was observed for group R but not for group U. The time course of recovery for operant and reflex responses clearly differed, indicating that different processes determined changes in these spinal and supraspinal reactions to nociceptive stimulation.