Failure of pinealectomy or melatonin to alter circadian activity rhythm of the rat

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Abstract
If the pineal gland is involved in the physiological mechanism by which the rat alters its free-running period (.tau.) in response to changes in illuminance was determined. Spontaneous wheel-running activity was recorded from pinealectomized or sham-operated female Charles River rats. The .tau. of running activity was determined in continuous darkness (DD) or in continuous dim light (LL). Pinealectomized rats and sham-operated rats lengthened their .tau. to approximately the same extent when shifted from DD to LL and shortened their .tau. when shifted back to DD. Continuous melatonin administration via Silastic capsules failed to alter .tau. of rats kept in dim LL. The pineal apparently is not primarily involved in the mechanism by which the rat alters .tau. in response to changes in illuminance.