The activities of Na-K-ATPase and Na-K-2Cl cotransporter (NKCC1) were studied in the aorta, heart, and skeletal muscle of streptozotocin (STZ)-induced diabetic rats and control rats. In the aortic rings of STZ rats, the Na-K-ATPase-dependent ⁸⁶Rb/K uptake was reduced to 60.0 ± 5.5% of the control value (P < 0.01). However, Na-K-ATPase activity in soleus skeletal muscle fibers of STZ rats and paired control rats was similar, showing that the reduction of Na-K-ATPase activity in aortas of STZ rats is tissue specific. To functionally distinguish the contributions of ouabain-resistant (₁) and ouabain-sensitive (₂ and ₃) isoforms to the Na-K-ATPase activity in aortic rings, we used either a high (10³ M) or a low (10⁵ M) ouabain concentration during ⁸⁶Rb/K uptake. We found that the reduction in total Na-K-ATPase activity resulted from a dramatic decrement in ouabain-sensitive mediated ⁸⁶Rb/K uptake (26.0 ± 3.9% of control, P < 0.01). Western blot analysis of membrane fractions from aortas of STZ rats demonstrated a significant reduction in protein levels of ₁- and ₂-catalytic isoforms (₁ = 71.3 ± 9.8% of control values, P < 0.05; ₂ = 44.5 ± 11.3% of control, P < 0.01). In contrast, aortic rings from the STZ rats demonstrated an increase in NKCC1 activity (172.5 ± 9.5%, P < 0.01); however, in heart tissue no difference in NKCC1 activity was seen between control and diabetic animals. Transport studies of endothelium-denuded or intact aortic rings demonstrated that the endothelium stimulates both Na-K-ATPase and Na-K-2Cl dependent ⁸⁶Rb/K uptake. The endothelium-dependent stimulation of Na-K-ATPase and Na-K-2Cl was not hampered by diabetes. We conclude that abnormal vascular vessel tone and function, reported in STZ-induced diabetic rats, may be related to ion transport abnormalities caused by changes in Na-K-ATPase and Na-K-2Cl activities.