1 The PI-purinoceptors which mediate the inhibition by adenosine of nerve-mediated contraction of the rat vas deferens have been investigated by use of the agonists N6-cyclopentyladenosine (CPA) and 5'-N-ethylcarboxamidoadenosine (NECA) and the Al-selective antagonist 1,3-dipropyl-8-cyclopentylxanthine (DPCPX). The ontogeny of the responses to adenosine and to the two co-transmitters which induce the contractions in this tissue, adenosine 5'-triphosphate (ATP) and noradrenaline (NA), have also been studied. 2 The order of potency for the adenosine agonists in inhibiting the nerve-mediated contractions was CPA = NECA> adenosine. Micromolar concentrations of DPCPX were required to antagonize the inhibition by adenosine and NECA of nerve-mediated responses, whereas the inhibitory effect of CPA was antagonized by nanomolar concentrations of the antagonist. 3 NECA and adenosine inhibited contractions induced by ATP (1OIM) or by NA (1OIM), NECA being at least ten fold more potent than adenosine, whereas CPA was inactive. Micromolar concentrations of DPCPX were required to antagonize the effect of adenosine on the contractions induced by ATP (1OIM). 4 Nerve-stimulated contractions could be observed in neonatal tissues from day 15 and increased with age, and could be inhibited by adenosine from this time, the potency of adenosine decreasing with age. Responses to ATP also appeared at day 15 and increased with age up to day 25, while responses to NA were present from day 10 (the earliest day tested) and decreased with age.5 These results show that the rat vas deferens contains both prejunctional Al-receptors and postjunctional A2-receptors, and that adenosine acts on the latter populations to inhibit nerve-mediated contractions. The high potency of adenosine in the neonate and the parallel development of responses to ATP and to nerve-mediated contractions support suggestions that purinergic responses may be particularly important in neonatal tissues.