Transient changes in retinal oxygen in response to light stimuli were studied to further understand the light-evoked change in oxygen consumption. Double-barreled microelectrodes, which measured oxygen and local voltage simultaneously, were positioned near the photoreceptor inner segments of the toad neural retina-retinal pigment epithelium-choroid preparation. Light-evoked oxygen responses were measured in a normal [Na+] solution, and in a test solution with lowered extracellular [Na+] to inhibit Na+/K+ pumping. Under the normal [Na+] condition, retinal oxygen tension increased in response to light indicating that oxygen utilization had decreased. When the Na+ concentration was lowered in the retina, the oxygen tension decreased in response to light, indicating an increase in oxygen utilization which was smaller than the Na+/K+ pump effect and therefore masked under normal conditions. The increase in oxygen utilization in lowered [Na+] was suppressed by adding 0.7 mM 3-isobutyl-1-methyl-xanthine, a phosphodiesterase inhibitor, suggesting that the response was largely due to hydrolysis and subsequent resynthesis of cyclic GMP. Results of fitting the light-evoked responses to exponential functions suggested that the decrease in oxygen consumption caused by slowing of the photoreceptor Na+/K+ ATPase had a time constant between 130 and 180 sec and that the increase in oxygen utilization from increased cyclic GMP synthesis was faster.