Outer medullary descending vasa recta (OMDVR) were dissected from the outer medullary vascular bundles of young rats, perfused in vitro, and loaded with fura 2 for measurement of intracellular calcium concentration ([Ca2+](i)) by fluorescent ratio imaging. Fluorescent video images revealed that fura 2 selectively loads into endothelial cells but not pericytes. Bradykinin (BK), at concentrations >10(-11) M, elicited an increase in [Ca2+](i) from baseline values in the range from 50 to 100 nM to peak values of 600-800 nM followed by a sustained plateau of 150-250 nM. The vasopressin V-1-receptor agonist [Phe(2),Ile(3),Orn(8)]vasopressin constricted OMDVR but yielded no observable [Ca2+](i) response, a finding that is consistent with an endothelial cell origin for the fura 2 fluorescent signal. The BK [Ca2+](i) response was blocked by the selective BK B-2-receptor antagonists D-Arg-[Hyp(3),Thi(5,8),D-Phe(7)]BK and D-Arg-[Hyp(3),D-Phe(7),Leu(8)]BK but not the B-1 antagonist des-Arg(9)-[Leu(8)]BK. BK vasodilated microperfused OMDVR that had been preconstricted with 10(-8) M angiotensin II. We conclude that the [Ca2+](i) response of OMDVR endothelia can be selectively studied with fura 2, that BK increases endothelial [Ca2+](i) via the B-2 receptor, and that BK can vasodilate descending vasa recta.