1. An electrophysiological analysis has been made of the storage and release of noradrenaline (NAd) in the sympathetic nerve terminals of the isolated vas deferens of the mouse. The amplitude of the excitatory junction potentials (e.j.p.s) recorded intracellularly in smooth muscle cells was taken as a measure of the NAd output per impulse from the terminals of sympathetic axons.2. During short trains of impulses (< 100), the amplitude of the e.j.p. increased with successive impulses at the beginning of a train, and then either continued to increase until a steady-state amplitude was reached (frequencies < 1 Hz), or decreased until a depressed steady amplitude was reached (frequencies > 1 Hz).3. During trains of impulses lasting for several minutes, the amplitude of the e.j.p. continually declined (frequencies > 1 Hz) until a steady-state amplitude was reached after 8 min of stimulation. This steady-state amplitude is smaller, the higher the frequency of stimulation.4. During short trains of impulses in the presence of high magnesium solutions, the amplitude of successive e.j.p.s increased until a steady state was reached, no matter what the frequency of stimulation. This growth of the e.j.p. amplitude during a train could be quantitatively predicted in terms of the linear summation of the individual facilitatory effects introduced by each impulse in the train.5. During trains of impulses lasting for several minutes, in the presence of a NAd synthesis blocker, the amplitude of the e.j.p. continually declined along a curve which could be described as the sum of two exponential components: one with a time constant of 1 min and the other of 10 min.6. These results suggest that NAd is released from a small pool of transmitter in sympathetic nerve terminals, which is replenished from two stores, which are in turn replenished by the synthesis of new NAd.