The effect of ethanol on behavioral thermoregulation in the goldfish, Carassius auratus, was studied by adding ethanol to a horizontal aquatic temperature gradient which allowed each fish to select its preferred temperature within a range of about 9 degrees C to 33 degrees C. Alternating exposure to 1.0% (v/v) ethanol and water showed that fish (10 to 15 g) responded to ethanol by selecting lower temperatures. Onset and disappearance of the effect occurred within 10 min of exposure to or removal from ethanol. Fish exposed to 1.0% ethanol for 3 hr did not show acute tolerance. When fish were exposed to increasing concentrations of ethanol from 0.0% to 1.7%, the lowest concentration to elicit a response was 0.5% ethanol. The magnitude of the response plateaued at 0.7% ethanol. At this concentration and above, selected temperatures remained about 2 degrees C below temperatures selected by controls. Because thermoregulatory responses of fish are behavioral and relatively easy to observe and quantify, goldfish offer a useful model for the study of ethanol effects on central nervous system control of thermoregulation. Ethanol produces a prompt, stable, and reproducible depression of selected temperature by lowering the thermoregulatory set point in the goldfish.