The phase relationship between the circadian rhythms of locomotor activity and circulating melatonin levels under LD 12:12 and constant dim light conditions (LLdim) was examined in pigeons. Our new radioimmunoassay technique had sufficient sensitivity to detect the concentration of melatonin from a small quantity of plasma, which enabled us to measure locomotor activity simultaneously with circulating melatonin rhythms from each individual pigeon. The relationship of phase between the two circadian rhythms in LLdim was the same as in LD. The rhythms of locomotor activity and melatonin levels free-run but maintain the relation of 180 degrees out of phase in LLdim. These results do not contradict the hypothesis that melatonin may entrain the circadian rhythm of locomotor activity in the pigeon.