The amygdala plays a key role in emotional processing and anxiety-related physiological and behavioral responses. Previous studies have shown that injections of the anxiety-related neuropeptide corticotropin-releasing factor or the related neuropeptide urocortin 1 into the region of the basolateral amygdaloid nucleus induce anxiety-like behavior in several behavioral paradigms. Brainstem serotonergic systems in the dorsal raphe nucleus and median raphe nucleus may be part of a distributed neural system that, together with the basolateral amygdala, regulates acute and chronic anxiety states. We therefore investigated the effect of an acute bilateral injection of urocortin 1 into the basolateral amygdala on behavior in the social interaction test and on c-Fos expression within serotonergic neurons in the dorsal raphe nucleus and median raphe nucleus. Male rats were implanted with bilateral cannulae directed at the region of the basolateral amygdala; 72 h after surgery, rats were injected with urocortin 1 (50 fmol/100 nl) or vehicle (100 nl of 1% bovine serum albumin in distilled water). Thirty minutes after injection, a subgroup of rats from each experimental group was exposed to the social interaction test; remaining animals were left in the home cage. Two hours after injection rats were perfused with paraformaldehyde and brains were removed and processed for immunohistochemistry. Acute injection of urocortin 1 had anxiogenic effects in the social interaction test, reducing total interaction time without affecting locomotor activity or exploratory behavior. These behavioral effects were associated with increases in c-Fos expression within brainstem serotonergic neurons. In home cage rats and rats exposed to the social interaction test, urocortin 1 treatment increased the number of c-Fos-immunoreactive serotonergic neurons within subdivisions of both the dorsal raphe nucleus and median raphe nucleus. These results are consistent with the hypothesis that the basolateral amygdala and serotonergic neurons within the midbrain raphe complex are part of an integrated neural system modulating anxiety state.