Pinealectomized and intact chicks raised under cyclic (14L:10D) and noncyclic (OL;24L) conditions of environmental lighting were sampled 60 hr and 6 weeks after pinealectomy to evaluate the effects of pineal ablation on plasma and ocular melatonin concentrations. Little or no immunoreactive melatonin was present in plasma 60 hr after pinealectomy, while appreciable amounts (38-70% of control values) were measurable 6 weeks later. We also found photoperiod to be an important variable in determining the relative amounts of melatonin present in the eyes and the pineal gland. At night or in continuous darkness, the eyes contain proportionately larger amounts of melatonin; by day, or in continuous light, they contain less. Finally, it was determined that pinealectomy significantly increases ocular melatonin concentrations (+ 62-80%) in chicks raised in the 14L:10D and 24L environments. These results show that compensatory mechanisms exist for maintaining circulating levels of melatonin in the absence of the pineal, and that they require some time to develop. Furthermore, there appears to be a dynamic interrelationship between the eyes and the pineal gland in terms of melatonin production. This is evidenced by the differential effects of environmental lighting on melatonin concentrations in these tissues and by the increased ocular concentrations of melatonin in pinealectomized animals.