The pineal organ of the lizard Anolis carolinensis acts as a transducer of photoperiodic information, since light can affect the pineal melatonin rhythm (PMR). The synthesis and secretion of melatonin may be a major mechanism whereby a circadian pacemaker within the pineal can control circadian clocks located elsewhere. An investigation into potential routes by which light could affect the PMR showed that (1) removal of the photosensory parietal eye did not affect the PMR as compared to controls under either a light-dark (LD) 12:12 cycle and a constant temperature (32 degrees C) or an LD 12:12 cycle and a daily temperature cycle (32 degrees C/20 degrees C); (2) removal of both the parietal eye and the lateral eyes did not affect the PRM of anoles held in LD 12:12 (constant 32 degrees C); (3) the PMR of blinded anoles re-entrained to a 10-hr shift in the phase of the LD cycle as rapidly as that of sighted anoles; (4) blocking light penetration to the brains of anoles, but leaving the lateral eyes exposed, blocked the ability of anoles to re-entrain to a 10-hr shift in the phase of an LD cycle. The data support the hypothesis that light directly affects the PMR in Anolis and that other potential photic inputs (parietal eye, lateral eyes) play little or no role. This conclusion is supported by previous neurophysiological and ultrastructural studies showing that the lizard pineal possesses functional photoreceptors.