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To study endothelial cell (EC)– specific Ca²⁺ signaling in vivo we engineered transgenic mice in which the Ca²⁺ sensor GCaMP2 is placed under control of endogenous connexin40 (Cx40) transcription regulatory elements within a bacterial artificial chromosome (BAC), resulting in high sensor expression in arterial ECs, atrial myocytes, and cardiac Purkinje fibers. High signal/noise Ca²⁺ signals were obtained in Cx40^BAC-GCaMP2 mice within the ventricular Purkinje cell network in vitro and in ECs of cremaster muscle arterioles in vivo. Microiontophoresis of acetylcholine (ACh) onto arterioles triggered a transient increase in EC Ca²⁺ fluorescence that propagated along the arteriole with an initial velocity of ≈116 μm/s (n=28) and decayed over distances up to 974 μm. The local rise in EC Ca²⁺ was followed (delay, 830±60 ms; n=8) by vasodilation that conducted rapidly (mm/s), bidirectionally, and into branches …