The efflux of sodium (24Na) was measured in the aortas from control and deoxycorticosterone (DOCA) hypertensive rats. Measurements were also made of aortic water content, extracellular space, and electrolyte contents. The objective was to determine the kinetics for potassium (K)-stimulated (active) transport of sodium and to determine which, if any, kinetic parameters are altered during DOCA hypertension. The K-stimulated efflux of 24Na (about 50% of the total) exhibited a sigmoidal dependence on extracellular K ([K]0), and agreed well with a model based on the cooperative interaction between Na and K at transport sites. Half saturation was achieved at [K]0 = 1.7 to 2.8 mM for the various groups. The maximum efflux was increased 40% to 50% by DOCA (p less than 0.01). Aortas from control and DOCA-treated rats had similar [Na]cell (11 mM/liter cell H2O, for physiological solution) under conditions of partial or complete Na loading. Active transport of Na showed a sigmoidal dependence on [Na]cell. Half saturation was achieved at [Na]cell = 20 to 24 mM/liter cell H2O in control and DOCA aortas. The maximum efflux was elevated 46% by DOCA treatment (p less than 0.01). It is concluded that DOCA hypertension is associated with an increased leak of ward of shift of the transport curves as a result of increased maximal transport. The net effect is to maintain [Na]cell relatively constant in DOCA hypertensives under physiological conditions.