The effects of morphine on the responses of blood pressure and pulse rate to stimulation of sympathetic nerves or to intravenous administration of noradrenaline were studied in female rats which had been treated with either an increasing concentration of morphine sulphate in their drinking fluid (5% sucrose solution), or an acute intraperitoneal injection of morphine. Sympathetic nerve excitation was effected by electrical stimulation of the thoracic segments of the spinal cord in pithed rats. Both sympathetic nerve stimulation and noradrenaline produced dose-dependent changes in blood pressure and pulse rate in naive rats and in the sucrose-drinking controls. Animals which had been chronically treated with morphine in their drinking fluid for 21 days showed significantly less intense responses to sympathetic nerve stimulation. However, these decreased responses were not observed in rats given acute treatment with morphine. Chronic treatment with morphine did not significantly influence the changes in blood pressure or pulse rate induced by noradrenaline. These findings suggest that chronic treatment with morphine lessens the cardiovascular responses to stimulation of peripheral sympathetic nerves in rats. The mechanism is not clear, but it seems unlikely to be due to changes in the sensitivity, or perhaps the number, of adrenoceptors.