Enhancement of serotonin neurotransmission plays an important role in the antidepressant response to agents presently available to treat depression. This response forms the major evidence for the role of serotonin in affective and social behaviour in humans. The present study investigated the effects of acute administration of the selective serotonin reuptake inhibitor (SSR1), citalopram (10 mg, i.v.) upon a measure of emotional processing in healthy female volunteers. Subjects completed a facial expression recognition task following infusion of citalopram or saline (between-subjects design, double-blind). Facial expressions associated with five basic emotions--happiness, sadness, fearfulness, anger and disgust--were displayed. Each face had been 'morphed' between neutral (0%) and each emotional standard (100%) in 10% steps, leading to a range of emotional intensities. Mood and subjective experience were also monitored throughout the testing session. Volunteers receiving citalopram detected a higher number of facial expressions of fear and happiness, with reduced response times, relative to those given the placebo. By contrast, changes in the recognition of other basic emotions were not observed following citalopram. Notable differences in mood were also not apparent in these volunteers. These results suggest that acute administration of antidepressant drugs may affect neural processes involved in the processing of social information. This effect may represent an early acute effect of SSRIs on social and emotional processing that is relevant to their therapeutic actions.