1 Intracellular recordings were made from smooth muscle cells of vasa deferentia in vitro. Vasa from two groups of mice were studied; the first were naive and the second had been chronically pretreated with morphine for 3 days. The vasa from morphine-pretreated mice were maintained in Krebs solution containing normorphine (300 nM). 2 The resting membrane potentials of the smooth muscle cells were the same in both groups of mice. 3 The excitatory junction potentials (e.j.ps) evoked by stimulation of the intramural nerves were depressed by normorphine in both groups of mice. The EC50 for this action of normorphine was 560 nM for the naive group and 6.6 microM for the morphine-pretreated group. 4 The EC50 for adenosine in depressing e.j.p. amplitude was the same in the two groups. 5 Naloxone did not change the resting membrane potential in cells from either group of mice. In morphine-pretreated mice, naloxone caused a marked increase in the amplitude of the evoked e.j.p. 6 The EC50 for noradrenaline in causing a contractile response of the isolated vas deferens was the same in both groups of mice. 7 The results indicate that changes in postsynaptic sensitivity to transmitter do not occur following morphine pretreatment.