The net passive influx of Na+ and efflux of K+ (orthodirection) through the red blood cell membranes from spontaneously hypertensive rats (SHR) were observed to be significantly higher (p less than 0.05) than those of three strains of normotensive rats when the measurements were made at 4 degrees C. Similar comparative studies, carried out at 37 degrees C, in the absence or presence of ouabain, showed no difference in these fluxes through this membrane from SHR compared to those from Wistar-Kyoto (WKY) rats, one of the normotensive strains. A study was undertaken to determine the temperature at which the greater cation fluxes in SHR red blood cells occurred. The net fluxes of Na+ and K+ decreased as the temperature was reduced from 37 degrees to 15 degrees C, but a paradoxical increase in the fluxes was observed as the temperature was decreased from 15 degrees to 4 degrees C. Only with this temperature shift (15 degrees to 4 degrees C) was the increase in flux significantly greater in SHR than in WKY cells. Subsequent studies were designed to determine whether the difference in the transport systems of red blood cells of SHR and WKY could be observed in fluxes of these cations in either direction across the membrane. For "reverse direction" flux studies, red blood cells were loaded with Na+ (to 130 mEq/liter cell water) and depleted of K+ (to 30 mEq/liter cell water) by incubation with the ionophore monensin.(ABSTRACT TRUNCATED AT 250 WORDS)