The amount of transmitter in the electric organ of Torpedo was measured with a time resolution of 1 sec in the course of stimulation. In parallel, the modifications of the electrophysiological response were analysed by determining the conductance increase (deltaG) and the electromotive force of electroplaques. Large changes in the level of total acetylcholine (ACh) were seen during stimulation. These changes were two-fold: a slow wave and, superimposed on it, a rapid oscillation. The slow wave raised total ACh to the initial level, or even higher. It was probably related to modifications in the amount of ACh released since it corresponded to characteristic inflections in the evolution of the deltaG curve. The slow wave and this physiological parameter were similarly affected when the experiments were performed at a reduced temperature. The rapid oscillation had an amplitude of about 20-40% of the total ACh. It was undamped and its period was 4-5 sec. In contrast to the slow wave, no clear physiological change associated with the rapid oscillation has been observed. The slow wave and rapid oscillation occurred in the 'free pool' of ACh, whereas bound ACh, the fraction associated with synaptic vesicles, was not affected by these changes. A dynamic description of synaptic activity is proposed. The content of 'free' ACh is used and renewed completely after a few tens of impulses, so that transmission seems to imply the continual recycling of the same pool of transmitter rather than utilization of a large preloaded store. The release process must then be integrated in rapid metabolic loops.