There have been many reports of increased Na-H exchange (NHE) activity in the peripheral blood cells (erythrocytes, lymphocytes and platelets) of patients with diabetes mellitus compared to nondiabetic controls. This raised NHE activity has been hypothesized to reflect increased NHE activity in kidney and vascular smooth muscle. Raised NHE activity in these tissues could play a pathophysiological role in mediating hypertension, vascular smooth muscle cell proliferation and progressive renal impairment. It is now known that there are at least five NHE isoforms, but a specific study examining expression of NHE isoforms in peripheral blood cells has not been reported. This study used specific antisera to NHE isoforms 1, 3 and 4 to examine NHE expression by immunoblot analysis. Erythrocyte, lymphocyte and platelet membranes from both rabbit and rat were separated by standard methods. A monoclonal antibody to NHE-1 reacted with a 100-110 kDa band in rabbit and rat platelets and lymphocytes (identical to that observed in basolateral-enriched renal cortical vesicles) and a 100 kDa band in rabbit and rat erythrocytes. In both species, the intensity of the staining was greatest in platelet membranes. A polyclonal antibody to NHE-3, the isoform present on the apical membranes of renal proximal tubule, showed no evidence of staining in any of the peripheral blood cell preparations. Similarly there was no evidence of expression of NHE-4 in the peripheral blood cell preparations. Peripheral blood cells express NHE-1, which likely accounts for amiloride-sensitive Na-H exchange in these cells, playing a role in cell volume and pH regulation. However, there is no evidence that there is expression of NHE-3 or NHE-4 in peripheral blood cells. These data have implications for studies in hypertension and diabetes mellitus which measure peripheral blood cell Na-H exchange and hypothesize regarding a direct pathophysiological role for this increased activity.