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1. Electrophysiological techniques were used to observe the release of transmitter from single release sites in the sympathetic neuro-effector junction of the rodent vas deferens. Transmitter release produces transient peaks in the rate of depolarization of the smooth muscle cells, known as 'discrete events'. 2. The amplitude distributions of stimulus-evoked discrete events in both mouse and guinea-pig vas are multi-modal. In the mouse, the distribution fits a Poisson with a 'quantal content' of about two. There are too many zeros in the amplitude distributions of guinea-pig discrete events to fit a Poisson distribution, and it is likely that in this species there is a mechanism preceding the transmitter release process which may occasionally prevent it operating. 3. alpha-adrenoreceptor agonists and antagonists produce, respectively, left and right shifts in the amplitude distributions of discrete events at a single latency, with no change in the amplitudes at the modes. 4. There is, however, no evidence of any inhibitory relationship between either discrete events evoked by successive stimuli, or early and late discrete events following a given stimulus. 5. Transmitter release at this junction is therefore packeted, with few quanta released by each stimulus. Release from single sites is affected in ways compatible with the 'alpha-feed-back' hypothesis by alpha-adrenoreceptor agonists and antagonists, but in the absence of drugs we can find no evidence of any local feed-back inhibition of transmitter release.