Our recent studies have implicated both the eyes and pineal as major components of the circadian system of Japanese quail. We assessed the role of these organs by examining the effect of their removal on the circadian activity rhythm of quail exposed to either 24 hr light-dark (LD) cycles or to continuous darkness (DD). Removal of only the pineal had no effect on the activity rhythm of quail in either LD or DD. Blinding (by orbital enucleation) had a major effect under both LD and DD. One third of the blinded birds showed entrainment under LD although entrainment patterns were very variable, whereas two thirds of blinded birds were arrhythmic. All blinded plus pinealectomized birds were arrhythmic in LD as were all blinded and blinded plus pinealectomized birds in DD. Accordingly, effects of pinealectomy can be seen only when pinealectomy is combined with blinding. The fact that blinding disrupts circadian organization in both LD and DD indicates that the eyes must act as major components of the quail's circadian system. In view of the postulated role for melatonin, an indoleamine, in circadian systems, the eyes, pineal, and blood of quail were assayed for this compound. Robust daily rhythms in melatonin content were observed in all three tissues. The blood rhythm is due to secretion of melatonin into the vascular system by both the pineal and eyes. The ocular melatonin rhythm continued after sectioning of the optic nerve, was reentrainable to a shift in the phase of the LD cycle, and persisted for at least 2 days in DD. These data suggest that the eyes play a major role within the circadian system and support the hypothesis that circadian pacemakers may reside within the eyes of quail. The results are discussed in view of the findings of others in both quail and other avian species. A general model for circadian organization in birds is presented in which the eyes, the pineal, and the suprachiasmatic nuclei of the hypothalamus comprise major elements of a multioscillator circadian system.