Renin secretion from renal juxtaglomerular cells is controlled by the intrarenal blood pressure, sodium chloride load of the organism, sympathetic nerve activity, and a number of hormones. The mechanisms by which these parameters exert their effects are not well understood. However, it is reasonable to assume that they act via signal transduction systems, implying the role of second messenger molecules in the control of renin secretion. This article summarizes present knowledge about the cellular mechanisms of renin secretion. Moreover, current concepts about the mechanisms by which the blood pressure and the sodium chloride load could influence renin secretion are discussed.