The postganglionic sympathetic nerves of rabbit isolated hearts were stimulated with pulses delivered at 5 Hz and train durations of 1-5 s. Ethylketocyclazocine 0.01-1 mumol/l and fentanyl 1 and 10 mumol/l but not morphine 1 and 10 mumol/l, Met-enkephalin 1 and 4 mumol/l or D-Ala2, D-Leu5-enkephalin 0.5 and 5 mumol/l diminished the stimulation-evoked increase in heart rate. The effect of ethylketocyclazocine 0.1 mumol/l was antagonized by naloxone 1 and 10 mumol/l. In contrast, the effect of fentanyl was not changed by naloxone 10 mumol/l. Ethylketocyclazocine 0.03 and 1 mumol/l did not reduce the tachycardia elicited by exogenous noradrenaline. The results suggest that, under in vitro conditions, only presynaptic opioid kappa- but not mu- or delta-receptors inhibit the release of noradrenaline from the sympathetic neurones innervating the sinus node.