Inhibition of cardiovascular Na,K-pump activity has been shown to promote an increase in the contractile activity of myocardial and vascular smooth muscle and a consequent rise in blood pressure (BP). It has also been shown that vascular Na,K-pump activity and myocardial Na+K+ATPase activity [the energy source for active sodium (Na) and potassium (K) transport] are decreased in rats with various forms of low renin hypertension including rats with reduced renal mass-saline (RRM-saline) hypertension. In the present study, left ventricular Na+K+ATPase activity from rats with RRM-saline hypertension was found to be decreased in membranes prepared by two independent methods: deoxycholate, sodium iodide (Nal)-treated microsomal fractions (method 1) and membranes prepared by the hypotonic, lithium bromide (LiBr) method (method 2). Relative to RRM normotensive control rats which drank distilled water, myocardial Na+K+ATPase activity from RRM-saline drinking rats was decreased by 18.2% in membranes prepared by method 1 and 33.6% in membranes prepared by method 2. The apparent affinities of Na+K+ATPase for K and for ouabain were unaltered relative to controls in membranes prepared from these hypertensive rats by method 1, and the sialic acid content and 5'-nucleotidase activity (two putative sarcolemmal markers) were unaltered in membranes from the hypertensive rats, prepared by methods 1 and 2 respectively. The Mg2+ATPase activity of membranes prepared by method 1 was increased in the RRM-saline hypertensive rats but because it was not increased in membranes prepared by method 2 the former observation does not appear to be of any pathophysiological importance. In other experiments, hypertension was reversed in RRM-saline hypertensive rats by restricting their salt intake (substitution of distilled water for drinking).(ABSTRACT TRUNCATED AT 250 WORDS)