The intrarenal distribution of renin in the mouse kidney was evaluated in a semiquantitative immunocytochemical study using an antiserum against pure mouse renin and the PAP technique. The bulk of renin positive cells was found in the media of the afferent arteriole. When examining the geometry of renin distribution about 35% of the afferent vessels were seen to be renin positive only over a distance of 20 micrometer, about 60% over a distance of 30 micrometer. In the remaining afferent arterioles, renin was also found upstream over distances up to 100 or even 200 micrometers. These results are discussed with regard to the stimuli for renin secretion, especially the macula densa signal. - At the vascular pole of the glomerulus, virtually 100% of the afferent, and 20%-40% of the afferent arterioles were found to be renin positive at an antiserum dilution of 1:1,000. As some efferent vessels - especially those of the juxtamedullar region - show scattered activity occasionally over a distance of more than 100 micrometer, it is suggested that the figure of 20%-40% should be taken as a minimal count for renin positive efferent arterioles. - To compare the renin content of superficial and juxtamedullary, afferent and efferent arterioles in normal salt and salt depleted mice, the fraction of positive renin reactions close to the vascular pole was determined at antiserum concentrations of 10(-3), 10(-4), 2 X 10(-4) and 10(-5). By this semiquantitative immunocytochemical method the afferent arterioles of superficial glomeruli could be shown to contain significantly higher renin concentrations than those of juxtamedullar glomeruli. This result was in agreement with biochemical renin estimations in mouse kidney slices taken from cortical and juxtamedullar sites. Sodium deprivation was followed by only a slight elevation of the fraction of positive superficial afferent arterioles (confirmed by the biochemical data). In contrast, sodium deprivation induced a highly significant increase of the number of positive superficial efferent vessels. This result is discussed with regard to (controversial) reports on a preferential efferent vasoconstrictor tone sustained by angiotensin II especially under the condition of sodium depletion. Juxtamedullar vasa afferentia and efferentia did not respond significantly to sodium restriction. - The Goomaghtigh cell field was found to be renin negative in superficial as well as in juxtamedullar glomeruli both in normal salt and salt deprived mice. Inspecting nearly 5,000 glomeruli, only 5 clearly renin positive mesangial cells were seen close to the glomerular stalk. In contrast, renin positive media cells could not seldom be seen in interlobular arteries and at the point of their branching into afferent arterioles.