To determine whether insulin regulates vascular smooth muscle Na Β§ , K Β§ activity and if impaired insulin stimulation of vascular smooth muscle Na +, K +-ATPase activity could be a cause of increased vascular reactivity to norepinephrine and angiotensin II in diabetic states, the effects of insulin on Na +, K +-ATPase activity were examined in normal rabbit aortic intima-media incubated with normal plasma glucose and rnyo-inositol levels for 30 min. Insulin at 100 gU/ml (600 pmol/1) had no effect on Na+, K +-ATPase activity. At 250 ~tU/ml it caused a 4.2 + 0.8 % increase, and at 500 gU/ml insulin caused a 17.7 + 1.4 % increase in Na +, K + -ATPase activity that was completely inhibited by amiloride (1 mmol/1). Human insulin-like growth factor I (600 pmol/1) caused an 18.0 _+ 1.0 % increase in Na', K +-ATPase activity that was inhibited by amiloride. Insulin does not regulate (stimulate) aortic vascular smooth muscle Na Β§ K+-ATPase activity. Supraphysiological insulin concentrations, probably acting through an insulin-like growth factor I receptor, stimulate Na+/H + exchange in aortic vascular smooth muscle and cause small secondary increases in Na T, K Β§ activity. In aortic intima-media incubated with normal plasma glucose and myo-inositol levels, endogenously released adenosine stimulates and maintains a component of resting Na +, K +-ATPase activity and stimulates acute increases in activity when norepinephrine (1 gmol/1) or angiotensin II (100 nmol/1) is added. These adenosine-stimulated components of Na +, K +-ATPase activity are selectively inhibited when the medium glucose is raised to 30 retool/1 during a 30-rain equilibration and 30-rain incubation. Insulin (100 gU/ml) added during the incubation had no effect on the alterations in Na +, K Β§ activity induced by glucose at an elevated plasma level. Impaired insulin stimulation of vascular smooth muscle Na +, K'-ATPase activity is not a possible cause for alterations in vascular reactivity in diabetes.