Intense depolarizing stimuli induce the expression of the proto-oncogene c-fos which may be useful as a marker of neuronal activity. To determine if mild physical and behavioral stressors may also induce c-fos expression, we subjected rats to an unconditioned stressor (footshock) or a conditioned stressor (a tone previously paired with footshock) and measured c-fos mRNA levels in various brain regions using in situ hybridization. Removing rats from their home cage and exposing them to a tone was sufficient to cause increases in c-fos mRNA in several forebrain areas while further increases in c-fos occurred in the septum, cingulate cortex, and endopiriform nucleus in response to acute footshock stress. Both unconditioned and conditioned stressors increased c-fos mRNA levels in the locus ceruleus which correlated with stress-induced plasma corticosterone concentrations. Unconditioned footshock stress also increased c-fos mRNA in the hypothalamic paraventricular nucleus (PVN). However, neither conditioned nor unconditioned stressors induced c-fos in the PVN in rats which had been previously exposed to footshock. C-fos appears to be a sensitive marker for stress-responsive brain areas and may be important in mediating long-term neurochemical changes that result from stress.