To better understand the role of the neuropeptide oxytocin in autism spectrum disorder (ASD), we investigated potential deficits in social play behaviour and oxytocin receptor (OXTR) density alterations in the amygdala in a rodent model of ASD. Pregnant rats were injected daily with 20 or 100 mg/kg valproic acid (VPA) or saline from day 12 until the end of pregnancy. The number of pinning and pouncing events was assessed at postnatal days 29–34. Brains from male offspring (n=7/group) were removed at postnatal day 50. We performed quantitative autoradiography with an OXTR radioligand, the [125I]-ornithine vasotocin analogue, in brain slices from the amygdala and other limbic brain regions involved in rat social behaviour. The results demonstrated a significant reduction in pinning behaviour and decreased OXTR density in the central nucleus of the amygdala in the 20 mg/kg VPA group. However, the 100 mg/kg VPA group had no significant changes in the number of play behaviour-related events or OXTR binding in the central nucleus of the amygdala. The reduction in OXTR density in the amygdala may be a critical disrupting mechanism affecting social behaviour in pervasive disorders such as ASD.