Noradrenaline (NA) evoked a contraction which consisted of two components, an initial one associated with only depolarization and a second one with spike discharges. Both contractions were abolished in a Ca-free solution leaving only small depolarization. In a Na-free solution, the initial contraction was increased in keeping with the short term depolarization, while the second contraction was abolished. It is suggested that NA initially causes Ca influx which secondarily increases Na permeability.