The expression of the Fos proto-oncogene protein has been used as an anatomical marker of activated brain areas. Detection of Fos immunoreactivity can provide information about the sites of action of various stimuli at the level of single cell resolution. Following intraperitoneal injection of ethanol (16% w/v), Fos immunoreactivity was induced in several rat brain areas including the bed nucleus of the stria terminalis, paraventricular hypothalamic nucleus, the central nucleus of amygdala, Edinger-Westphal nucleus, locus coeruleus nucleus and parabrachial nucleus. The induction was dose dependent, and the length of activation time was different in each nucleus. Fos immunoreactivity in the supraoptic nucleus appeared only when a higher concentration of ethanol was injected. Repeated administration of ethanol twice daily for 17 or 24 days resulted in a desensitization of Fos immunoreactivity in these nuclei. These data suggest that induction of Fos immunoreactivity can be used to determine the sites at which ethanol acts on the brain, and may provide important information about the mechanisms underlying the tolerance and physical dependence of alcohol usage.