Zebrafish have been shown to have preference for light or dark environments depending on the ambient light level and the presence or absence of food odor. We used a cylindrical tank, half of which was surrounded by a black surface and the other half by white, to elicit a choice from individual wild-type, adult zebrafish. One treatment group was exposed to food odor and the other to water (as a control) at the beginning of the trial. During 10-min trials, the light level was increased each minute over a fivefold range in steps from 1.34 x 10(17) photons/s/m(2) at the beginning to a final light level of 8.31 x 10(17) photons/s/m(2). We demonstrate that the preference of the zebrafish for the light or dark half of the cylinder is dependent upon ambient light levels as well as olfactory stimulation. These results provide a potential explanation for the contradictory observations that, when given a choice, adult zebrafish prefer brighter light environments (Gerlai et al., 2000) or darker light environments (Serra et al., 1999). Thus, we present data useful in designing more powerful and reliable behavioral assays for use with zebrafish as well as further information about the effect of olfactory stimulation on zebrafish visual behavior.