Electrophysiological techniques were used to observe the release of transmitter from one or a few release sites of the sympathetic neuroeffector junction of the mouse vas deferens. Release produces transient accelerations of the depolarizing phase of the excitatory junction potential, known as 'discrete events'. Discrete events associate into families at a constant latency and peak time, but vary in amplitude between a few preferred values. As facilitation develops there is a decrease in the frequency of small members of families and an increase in the frequency of large members, a change in apparent 'quantal content'. A similar change in amplitude distribution occurs when the [Ca]o is raised. The alpha-adrenoceptor antagonist yohimbine increases quantal content when facilitation has developed, but has no significant effect on unfacilitated discrete event amplitude unless the [Ca]o is below 2.0 mM. The alpha-adrenoceptor agonist clonidine reduces facilitated and unfacilitated quantal content under all conditions examined.