The response latency of golden hamsters to nociceptive stimuli was measured under cyclic lighting conditions and during constant illumination. A day-night rhythm of nociception was demonstrated; response latencies were significantly longer during the day. A circadian rhythm of nociception was displayed by hamsters maintained for 30 days in constant dim light. Short response latencies noted under these conditions were associated with the inactive period of the animals circadian cycle (subjective day). The experiments provide data which indicate the phase relationship between the circadian rhythms of nociception and locomotor activity differs under entrained and free-running conditions.