This study explores the role of K+ and aldosterone in the regulation of mRNA of the ATP-sensitive, inwardly rectifying K+ channel, ROMK, in the rat kidney. K+ deficiency downregulated ROMK mRNA in cortex to 47.1 +/- 5.1% of control (P < 0.001) and in medulla to 56.1 +/- 3. 4% (P < 0.001). High-K+ diet slightly increased ROMK mRNA in medulla to 122 +/- 9% (P < 0.05 vs. control). Adrenalectomy (Adx) downregulated cortical ROMK mRNA to 30.7 +/- 6.8% (P < 0.001 vs. control), and increased it in medulla to 138 +/- 12.9% (P < 0.02 vs. control). In Adx rats, K+ deficiency decreased ROMK mRNA in cortex and medulla similar to intact rats. The alpha1- and beta1-Na-K-ATPase subunits were regulated in parallel to that of ROMK. In medulla, ROMK mRNA correlated with serum K+ concentration at R = 0.9406 (n = 6, P < 0.001) and alpha1-Na-K-ATPase mRNA at R = 0.9756 (n = 6, P < 0.001). ROMK2 also correlated with serum K+ concentration (R = 0.895; n = 6, P < 0.01). These results show that cortical ROMK expression is regulated by aldosterone and K+, whereas the medullary ROMK mRNA is regulated by serum K+.