Plasma renin activity (PRA) was studied serially for up to 21 days following transplantation in thirteen patients receiving renal allografts. PRA was measured during fasting and recumvency and its relationship to renal function, diuretic administration, plasma sodium, allograft rejection and blood pressure was examined. PRA fell steadily as renal function improved and plasma sodium rose following transplantation and when rejection episodes were excluded an inverse relationship between PRA and renal function could be seen. It is not possible to say whether the changes in PRA and function are causally related or whether changes in plasma sodium alone account for the observed changes in PRA. Some rejection episodes were accompanied by an increase in PRA, but this was not sufficiently consistent to be of value in the diagnosis of rejection. PRA also increased in relation to frusemide-induced fluid loss. There was no relationship in these patients between PRA and blood pressure.