1. We measured forearm blood flow during brachial artery infusions of vasoconstrictors (angiotensin II and noradrenaline) and vasodilators (sodium nitroprusside and carbachol) in 16 healthy control subjects and in 18 patients with uncomplicated insulin-dependent diabetes mellitus. Erythrocyte Na + /Li + countertransport and platelet Na + /H + antiport activities were also measured. 2. The mean basal forearm vascular resistance was 22% lower in diabetic patients than in control subjects. The effects of each infusion on forearm vascular resistance were similar in diabetic patients and control subjects. 3. Erythrocyte Na + /Li + countertransport activities were similar in diabetic patients and control subjects. Platelet Na + /H + exchange ( V max. ) was approximately 40% greater in diabetic patients than in control subjects, whereas the K m for Na + was similar. 4. In diabetic patients, but not in control subjects, the responses to sodium nitroprusside and carbachol correlated inversely with Na + /Li + countertransport in erythrocytes ( r s = −0.76, P < 0.001, and r s = −0.66, P < 0.005, respectively), but not with Na + /H + exchange in platelets.