Leu-enkephalin (LENK) injected intracerebroventricularly (icv) in a dose of 50-400 micrograms to male Wistar rats inhibited their open field locomotor activity. This effect was antagonized by 1 microgram icv of ICI 174864 but not by naloxone (NAL). Doses of 5 and 10 micrograms of ICI 174864 icv induced a characteristic abnormal behavioral syndrome, which was inhibited by icv LENK, but not by morphine. ICI 174864 potentiated the analgesic effect of morphine, whereas NAL inhibited it. The results indicate that ICI 174864 is an antagonist of central delta-opioid receptors. The abnormal behavior induced by higher doses of ICI 174864 is suggested as an experimental model for studying the central delta-opioid receptors.