SYNCHRONIZATION OF CIRCADIAN-RHYTHMS OF HOUSE SPARROWS BY ORAL MELATONIN - EFFECTS OF CHANGING PERIOD

House sparrows (Passer domesticus) whose circadian rhythms of locomoto r activity and feeding had been abolished by pinealectomy were held in constant dim light and periodically exposed to melatonin in the drink ing water. By alternating 8 h of melatonin water with variable phases of tap water, rhythms with periods (T) ranging from 21 to 27 h were pr oduced. When melatonin was administered in rhythms with periods of 23, 24, and 25 h, feeding and locomotion behavior of most birds were rhyt hmic and synchronized with the exogenous melatonin rhythm. The rest ph ase coincided approximately with the phase of melatonin availability. Under melatonin cycles < 23 h and > 25 h, fewer birds had synchronized rhythms. Nonsynchronized birds were either arrhythmic or they express ed free-running rhythms. Under melatonin rhythms with periods between 23 and 26 h, the phase-angle difference between defined phases of the behavioral rhythms and the melatonin rhythm became more positive with increasing T. These data are consistent with the hypothesis (a) that p eriodic exogenous melatonin can substitute, at least to a certain degr ee, for the endogenous plasma melatonin rhythm normally resulting from the periodic melatonin secretion by the pineal gland, and (b) that th is melatonin rhythm acts on another oscillator, possibly the SCN, as p art of the overall circadian pacemaking system.