The changes of Na,K-ATPase activity and its regulation have been investigated in the renal cortex of 1-clip-1-kidney hypertensive rat. Ouabain-sensitive Na,K-ATPase activity (Emax) and [3H]ouabain-binding site (Bmax) in the hypertensive rat were slightly increased than those in the control. The levels of Na,K-ATPase alpha 1- and beta 1-subunit mRNA of the renal cortex in hypertensive rat were more increased than those in the control. Their increases were repressed by actinomycin-D, but not altered or more increased by cycloheximide. These results suggest that the increase of Na,K-ATPase activities and ouabain binding sites in 1-clip-1-kidney hypertensive rat may be correlated with the increases of gene expression in transcription level and/or of mRNA stability of Na,K-ATPase.