The aim of the study was to investigate whether the adrenal renin- angiotensin system plays an independent role in the regulation of mineralocorticoid biosynthesis in the adrenal gland and to explore the mechanisms of this action. Twelve-week-old male Sprague-Dawley rats were studied: 22 rats were maintained on a regular diet; 27 and 22 rats received a low salt diet with and without treatment, respectively, with the angiotensin II (Ang II) AT₁-subtype receptor antagonist losartan (10 mg/kg per day). A fraction of each group of rats underwent bilateral nephrectomy (n=12, 15, and 10, respectively) and was killed 48 hours later. In an additional group of 24 (12 intact and 12 nephrectomized) rats, the effects of the Ang II AT₂- subtype receptor antagonist PD123319 were investigated. In intact rats, plasma renin activity (PRA) and adrenal renin activity and expression were progressively raised by salt restriction and losartan, whereas aldosterone synthase mRNA and plasma aldosterone (PA) levels were increased by salt restriction and reduced by losartan. Forty-eight hours after nephrectomy, PRA fell to undetectable levels; in contrast, adrenal renin expression, assessed by semiquantitative reverse-transcriptase polymerase chain reaction (using GAPDH as a standard for gene expression), showed an 18-fold increase and was further increased after salt restriction and losartan (all P<.05). Also, adrenal renin activity was raised after nephrectomy and further increased after salt restriction (P<.05) and losartan. Cytochrome P450 aldosterone synthase expression in the adrenal cortex was stimulated by nephrectomy alone and by nephrectomy combined with low salt intake (P<.05), with consequent increases in PA concentrations. In losartan-treated salt-restricted nephrectomized rats, cytochrome P450 aldosterone synthase expression (P<.05 versus nephrectomy alone and nephrectomy plus salt restriction) and PA concentrations were diminished (P<.05) in spite of the observed increases of adrenal renin expression. The AT₂-receptor antagonism did not significantly affect PRA, adrenal renin, and aldosterone biosynthesis and production in either intact or nephrectomized salt-restricted rats. These results demonstrate that the adrenal renin-angiotensin system plays an independent role in the regulation of mineralocorticoid biosynthesis in vivo. This action is mediated primarily via the Ang II AT₁-subtype receptors.