Retinal and extraretinal photoreceptor organs involved in synchronization of nocturnal locomotor activity under 12-h light:12-h dark cycle (LD12:12) were investigated in catfish, Silurus asotus. Catfishes studied were: (1) intact; (2) pinealectomized; (3) blinded; (4) blinded and pinealectomized; and (5) blinded, pinealectomized, with the brain region covered with aluminum foil. When the light intensity was reduced during the light phase of the LD cycle, the threshold for 24-h rhythmicity was determined, by observing the synchronization to the given LD cycle with periodograms. Intact catfish showed almost the same threshold intensity (in the order of 10(-3) microW/cm2) as that of either blinded or pinealectomized animals. This suggests that information is combined through both organs and is related to the 24-h rhythm of locomotor activity at lower intensities. However, the animal without the eye and pineal organ could display 24-h rhythmicity at intensities higher by approximately 3 log units than those of the animals described above. This indicates the existence of extraretinal and non-pineal photoreceptors (ENPs), and the involvement of at least three types of photoreceptor in mediating nocturnal locomotor activity rhythms at higher intensities. One of the possible ENPs was assumed to be in the brain region.