The amygdala plays a key role in emotional processing and anxiety responses. Previous studies have shown that urocortin 1 (Ucn 1) injections into the basolateral nucleus of the amygdala (BLA) induce anxiety-like behavior. Brainstem serotonergic systems in the dorsal raphe nucleus (DR) may be part of a distributed neural system that, together with the BLA, regulates acute and chronic anxiety states. We therefore investigated the effect of an acute bilateral injection of Ucn 1 into the BLA on behavior in the social interaction test (SIT) and on c-Fos expression within serotonergic neurons in the DR. Male rats were anaesthetized with a combination of Diazepam (0.2 ml, I.P.) and Hypnorm (0.2 ml, I.M.), and implanted with bilateral cannulae into the BLA; 72 h after surgery, rats were injected with Ucn 1 (50 fmol/100 nl) or vehicle (100 nl of 1% bovine serum albumin in distilled water). Thirty minutes after injection, a subgroup from each experimental group was exposed to the SIT; remaining animals were left in the home cage. Two hours after injection rats were anesthetized with 0.5 ml Euthatal (200 mg/ml sodium pentobarbital, I.P.), perfused with paraformaldehyde and brains were removed and processed for immunohistochemistry. Acute injection of Ucn 1 reduced total interaction time in the social interaction paradigm without affecting locomotor activity or exploratory behavior. Ucn 1 treatment induced c-Fos immunoreactivity in serotonergic neurons within the dorsal part of the rostral DR (- 7.46 mm bregma) in home cage rats, and within the dorsal and ventral parts of the middle and caudal DR (-8.00 and -8.54 mm bregma, respectively) in rats exposed to the SIT. These results are consistent with the hypothesis that the BLA and topographically organized subpopulations of serotonergic neurons within the DR are part of an integrated neural system modulating anxiety state.