The cardiovascular effects of ethylketocyclazocine were studied in rats anaesthetized with pentobarbital and artificially ventilated. Ethylketocyclazocine (1 mg X kg-1 i.v.) induced a fall in heart rate and blood pressure. The bradycardia and the hypotension were not altered by bilateral vagotomy and atropine, but were inhibited by naloxone and Mr 2266 BS. Ethylketocyclazocine always induced bradycardia in beta-adrenoreceptor-blocked and pithed animals. This bradycardia was prevented by Mr 2266 BS. These results would suggest that a direct peripheral action may occur at kappa-sites, located on the heart after intravenous injection of ethylketocyclazocine in the rat.