Responses to light were recorded intracellularly from Limulus ventral photoreceptors during the application of diamide, a sulfhydryl oxidizing agent. At diamide concentrations of over 0.5 mM there was a brief phase of increased response followed by a marked decrease. Diamide also prolonged the response latency, and in several cases increased the frequency of spontaneous discrete waves. At concentrations of 0.25 mM and below, the response only increased. Although the underlying mechanisms of these effects are not understood, these findings may be significant for the understanding of transduction in photoreceptors.