1. In order to demonstrate whether modification of aldosterone secretion is mediated by parallel changes of K+ in the adrenal zona glomerulosa, the total (intracellular + extracellular) Na+ and K+ content of the rat adrenal cortex was determined with the electron microprobe.

2. Groups of rats were submitted to one of the following dietary regimens: standard, low Na+, high K+ or high Na+.

3. Distribution of Na+ and K+ across the zona glomerulosa and zona fasciculata was compared. Standards of known electrolyte concentration were also analysed.

4. The [Na+] was found to be greater in the zona glomerulosa than in the zona fasciculata but K+ was distributed evenly in both zones. This was independent of dietary regimen.

5. Aldosterone production, assessed by plasma aldosterone concentrations, could not be correlated with zona glomerulosa K+ content.

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