In recent years many investigators have reported the role of the sympathetic nervous system and the catecholamines in the regulation of renin secretion. It has been recently reported that plasma dopamine-beta-hydroxylase (DBH) excreted through the mechanism of the exocytosis with noradrenaline from the sympathetic postganglionic nervous endings is a mirror of the function of the sympathetic nervous system. In this investigation, blood pressure, urinary catecholamine excretion, plasma DBH activity and plasmsa renin activity (RPA) have been determined in normal individuals during the mecholyl test and insulin-induced hypoglycaemia. In the mecholyl test, plasma DBH activity and PRA markedly increased in five males who showed prominent responses of systolic blood pressure to mecholyl (S type) and changed slightly in eight males who showed minimal responses of systolic blood pressure to mecholyl (N type). It showed that there was a significant relationship between the maximal percent increases from their respective control values of plasma DBH activity and those of PRA following mecholyl. In insulin-induced hypoglycaemia, plasma DBH activity, blood pressure and PRA remarkably increased following a large dose of insulin administration (0.15 u/kg). In a dose of 0.1 u/kg, there was no significant increase of plasma DBH activity and blood pressure. PRA singificantly increased but was lower than a large dose of insulin. Also, it showed that their was a significant relationship between maximal percent increases from their respective control values of plasma DBH activity and those of PRA following insulin administration. These data indicate a close correlation between changes in plasma DBH activity and those of PRA during mecholyl and insulin-induced hypoglycaemia and lend additional support to the concept of sympathetic nervous system mechanism of renin secretion.