Angiotensin II and potassium are known to increase steroidogenic acute regulatory protein (StAR) protein levels. However, a corresponding increase in StAR mRNA levels has so far been observed only in response to angiotensin II. We therefore studied the regulation of adrenal StAR mRNA expression in the context of dietary potassium-stimulated aldosterone production. Wistar Kyoto rats (WKY) and spontaneously hypertensive rats (SHR) were fed a diet containing either 1% or 4% KCl for five days. The high-potassium diet increased StAR mRNA levels within the zona glomerulosa in both strains, as demonstrated by in situ hybridization. However, aldosterone production increased in WKY, but not in SHR (WKY: from 22.8 ± 4.8 to 137 ± 25 ng/100 ml P<0.001 vs. SHR: from 29 ± 3.8 to 51 ± 10.2 ng/100ml; n. s.). This increase was associated with an increase in Cyp11b2 mRNA levels in WKY (3-fold; P<0.001), but not in SHR. In both strains, the 4% KCl diet was associated with increased plasma renin-independent aldosterone production, as indicated by the marked increase of the aldosterone to renin ratios (from 1.4 ± 0.3 to 9 ± 3 in WKY and from 3 ± 1 to 14 ± 5 in SHR; P <0.002). Conclusion: An increase of StAR mRNA levels within the outer cortex is involved in the long-term adrenal response to potassium. This increase alone is not sufficient to increase aldosterone production in the presence of normal Cyp11b2 mRNA levels.