The electrical activity of longitudinal muscle of anterior mesenteric artery from domestic fowl was investigated with microelectrodes and in the sucrose-gap apparatus. Action potentials were recorded by both techniques. In the sucrose-gap each action potential preceded a phasic contraction and similar contractions were recorded in vivo suggesting that during life, action potentials are normally associated with the generation of tension. Measured with microelectrodes in hypertonic solution at 41 °C, the time constant of the muscle was 257 msec and the space constant 2.8 mm. The latter was unchanged when reducing the tem perature to 31 °C but the former was increase