Microinjections of dopamine (DA) were made into specific forebrain loci in goldfish (Carassius auratus: 40-85 g) to study the involvement of DA in behavioral thermoregulation. Injections of 25, 50, 100 and 250 ng DA into the anterior aspect of the nucleus preopticus periventricularis (NPP) led to consistent, dose-dependent decreases in selected temperature. Minor decreases or no effect on selected temperature was observed following injections of 5 or 10 ng DA. Injections of the control solution were without effect. Injections of DA into other forebrain loci, including the posterior half of the NPP, either had no thermoregulatory effect or had minor thermoregulatory effects which, in comparison, to injections into the most effective sites, were inconsistent and required larger doses to obtain. The decrease in selected temperature following injections of 100 ng DA into the anterior NPP was blocked by haloperidol, a dopaminergic antagonist, but not by phentolamine, a noradrenergic antagonist. Injections of haloperidol alone resulted in a minor, but statistically significant, increase in selected temperature. The most sensitive DA sites lie caudal to the sites most sensitive to norepinephrine within the anterior NPP. DA acts on the dopaminergic receptors of central thermoregulatory neurons in the anterior NPP of goldfish. These receptors appear to mediate behavioral responses to excessively warm environments.