As previously shown in many other photoreceptors, during prolonged exposure to light, receptor sensitivity (mv/photon catch) declines and the speed of the response (rate of depolarization and rate of repolarization) increases. The functional significance of internal and external calcium in these changes were examined in the crayfish. We show that the response time characteristics depend significantly on the internal calcium level, whereas sensitivity appears to depend on the extracellular calcium. The effect of BAPTA, a calcium chelator, within the photoreceptor was to slow the response kinetics without changing sensitivity. We also examined the role of potassium channel blockers in accounting for BAPTA's effect. Although these blockers displayed similar effects to that of BAPTA, the data fail to provide irrefutable evidence that calcium works through the activation or the deactivation of K\sp+ conductances alone. Probably (even exclusively), internal calcium also affects the transduction process that precedes the channels openings.