1 Excitatory junction potentials (e.j.ps) occurring spontaneously or evoked by nerve stimulation were recorded intracellularly from smooth muscle cells of the mouse isolated vas deferens. 2 The amplitude of the evoked e.j.ps and the amplitude and frequency of spontaneous e.j.ps were measured before and during application of normorphine or solutions which might be expected change the influx of calcium ions into the nerve terminals. 3 Spontaneous e.j.ps could be recorded even in solutins which contained tetrodotoxin (1 microM), no added calcium an EGTA (1mM). A four fold increase in calcium concentration from 1.25 to 5 mM greatly increased the amplitude of the evoked e.j.ps but had no effect on the amplitude or frequency of the spontaneous e.j.ps. 4 Magnesium (12mM) and cobalt (4mM) both greatly reduced the evoked e.j.ps and also reduced the frequency of spontaneous e.j.ps. 5 Normorphine (2 microM) reduced the amplitude of the evoked e.j.p by 70% but had no effect on the amplitude or frequency of spontaneous e.j.ps. 6 It is suggested that normorphine inhibits noradrenaline secretion from nerve varicosities by a mechanism different from that of magnesium and cobalt. One possibility is a block of action potential propagation along varicose fibers.