A series of adrenoceptor agonists were investigated for their prejunctional effects on field stimulated rat vas deferens. Tissues were stimulated in 10 s trains of impulses, frequency 10 Hz, every 100 s. This produced a biphasic response comprising an initial twitch followed by a prolonged, plateau phase of contraction. The order of potency for a series of alpha 2-agonists against the twitch phase of contraction was UK14304 greater than clonidine greater than noradrenaline = alpha-methyl noradrenaline greater than B-HT920. The same order of potency was observed against the plateau phase, but approximately 10 fold higher concentrations of agonist were needed. Surprisingly, B-HT920 was inactive against the plateau phase of contraction. Characteristic differences in the slopes and maximum responses of the dose-response curves to the imidazolines (UK14304 and clonidine) and the beta-phenethylamines (noradrenaline and alpha-methyl noradrenaline) were seen against both phases of contraction. It is concluded that the two phases of contraction are influenced by activation of two distinct heterogeneous populations of prejunctional alpha 2-adrenoceptors.