Absence of Extraocular Photoreception in Diurnal and Nocturnal Rodents Exposed to Direct Sunlight

l. Entrainment of circadian wheel running activity was monitored over a period of several months for northern grasshopper mice (Onychomys [eucogasrer) and golden mantled ground squirrels (Sperm&i/us lateralis) housed out-of-doors and exposed to direct sunlight (average light intensity = 55,000 Ix). 2. Sighted animals entrained their activity rhythms to the natural photoperiod; the mice were nocturnal and the squirrels diurnal in their locomotor activity. 3. The activity rhythms of all blind animals from each species failed to entrain to the light-dark cycle and free-ran with endogenous periods > 24 hr for squirrels and < 24 hr for grasshopper mice. 4. Failure of entrainment in blind ground squirrels and grasshopper mice indicates that these species lack functional extraocular photoreceptors; they thus differ from non-mammalian vertebrates in which extraocular photoreception is well documented. 5. The sirmificance of exclusive reliance bv mammals on ocular photoreceptors for entrainment of circadian &ythms is discussed.