Dopamine (DA) is an important neuromodulator in the visual system. The release of DA in the retina largely depends on environmental lighting conditions. Most previous studies have assessed the effect of illumination on retinal DA or its metabolites using homogenates or in vitro preparations. This study was designed to investigate the effect of transitions between lighting conditions--from dark to steady or flickering light and vice versa--on retinal DA release in zebrafish using in vivo microdialysis. The transition from dark to flickering light increased DA release, whereas the transition from flickering light to dark decreased it. This latter effect depended on time of day within the light period, e.g., it was strongest in the late afternoon. When using steady light, none of these effects were seen. Our study also demonstrates that in vivo microdialysis can successfully be applied to the investigation of retinal DA release in zebrafish.