Goldfish were either subjected to anoxia for 5 h at 10 degrees C, normoxia for 5 h at 10 degrees C, or were implanted with an intracranial microinjection cannula. All groups were subsequently tested in a temperature gradient. The previously anoxic goldfish selected cooler temperatures (12.8 +/- 1.0 degrees C; mean +/- SE) than the corresponding normoxic control group (17.5 +/- 0.7 degrees C) for the first 20 min in the gradient. Intracranial microinjections of 0.0475 ng ethanol in 0.2 microliter 0.7% NaCl led to the immediate selection of water 8.7 +/- 1.5 degrees C below that of base-line levels, whereas control animals injected with 0.7% NaCl selected water 0.9 +/- 1.0 degree C cooler. Increased effects were obtained with higher concentrations of ethanol. The effective site was limited to the anterior aspect of the nucleus preopticus periventricularis; injections into 54 other loci were without effect. Goldfish tolerate anoxia by the conversion of lactate to ethanol, which diffuses across the gills. As the lost ethanol cannot be oxidatively metabolized, this process is energetically inefficient. Because the concentration of the ethanol injections was considerably lower than reported ethanol concentrations in the tissues of anoxic goldfish, endogenously produced ethanol may have induced the selection of cooler water by the anoxic goldfish. This alteration in thermoregulatory behavior would lead to a lower metabolic rate, significantly increasing survival time during anoxia.