The aim of this study was to examine the influence of dietary NaCl intake on renin gene expression in the kidneys and adrenal glands of adult rats. Rats were kept on low (0.02%, w/w), normal (0.6%) or high (4%) NaCl diets and plasma renin activity (PRA) and the relative abundance of renin messenger ribonucleic acid (mRNA) in renal and adrenal tissue were followed for 20 days. In animals on a normal-salt diet PRA and renal renin mRNA levels did not change with time. PRA values in animals on the low-salt diet increased transiently (about threefold) and then declined again during the third week of treatment. Renal renin mRNA levels in these animals paralleled the changes of PRA. Conversely, in the animals kept on a high-salt diet PRA values decreased transiently and renal renin mRNA decreased continuously to about 50% of control values. Arterial blood pressure measured in conscious animals was not significantly influenced by the different salt diets. To establish whether the changes in renin mRNA levels are mediated by renal nerve input, animals on the different diets were also studied after unilateral renal denervation. Renal nerve section led to a 50% decrease of renin mRNA levels in the denervated kidneys in animals kept on the normal-salt diet. In the animals on the low-salt diet renin mRNA rose to similar levels in the denervated to those in the innervated kidney, while in animals receiving a high-salt diet renin mRNA was further decreased in the denervated kidneys. The abundance of renin mRNA in adrenal tissue was low and was estimated to be around 1% of that found in the kidneys. Adrenal renin mRNA levels also increased in animals kept on a low-salt diet and decreased in animals on high-salt diet. Taken together, our findings suggest that renin secretion and renin gene expression are inversely related to salt intake and that the influence of salt diet on these parameters has both transient and constant temporal components. Changes of blood pressure or nerve activity are not likely mediators of the effect of salt intake on renin expression. Since renal and adrenal renin mRNA levels change in parallel in response to alterations of salt intake we hypothesize the existence of a humoral factor that links renin expression to the rate of salt intake.