In isolated spontaneously beating right and left atria and in electrically driven left atrium from rat, endothelin-1 increased the rate and force of contraction, but significantly decreased the positive chronotropic and inotropic responses to sympathetic nerve stimulation. The decrease may be partly dependent on the positive cronotropic and inotropic effects of endothelin-1, since other agents with chronotropic activity (noradrenaline, isoprenaline, serotonin and Bay k 8644) also decreased stimulation-induced chronotropic responses. Endothelin-1 caused a significant rightward shift of the linear portion of the log concentration-response curve for the chronotropic actions of noradrenaline and isoprenaline. The changes in the log concentration-response curve were not a consequence of the direct chronotropic effect of endothelin-1, since they were still evident when the chronotropic action of endothelin-1 was offset by carbachol. Furthermore, the chronotropic agent, Bay k 8644, did not shift the linear portion of the log concentration-response curves for noradrenaline and isoprenaline. The mechanism of the effects of endothelin-1 in rat atria is not known, but they were not changed by blockade of alpha-adrenoceptors or of L-type voltage-sensitive Ca2+ channels.