The visual system adjusts its sensitivity to a wide range of light intensities. We report here that mutation of the zebrafish sdy gene, which encodes tyrosinase, slows down the onset of adaptation to bright light. When fish larvae were challenged with periods of darkness during the day, the sdy mutants required nearly an hour to recover optokinetic behavior after return to bright light, whereas wild types recovered within minutes. This behavioral deficit was phenocopied in fully pigmented fish by inhibiting tyrosinase and thus does not depend on the absence of melanin pigment in sdy. Electroretinograms showed that the dark-adapted retinal network recovers sensitivity to a pulse of light more slowly in sdy mutants than in wild types. This failure is localized in the retinal neural network, postsynaptic to photoreceptors. We propose that retinal pigment epithelium (which normally expresses tyrosinase) secretes a modulatory factor, possibly L-DOPA, which regulates light adaptation in the retinal circuitry.