1. Field stimulation of the sympathetic nerves of the guinea-pig isolated vas deferens with trains of pulses of 20 s at 1-8 Hz produced characteristic biphasic contractions. The effect of the novel ecto-ATPase inhibitor, 6-N,N-diethyl-D-beta, gamma-dibromomethyleneATP (ARL 67156, formerly known as FPL 67156), on the magnitude of the initial, predominantly purinergic peak of this response was studied in order to determine the influence of enzymatic degradation of adenosine 5'-triphosphate (ATP) on its action as a neurotransmitter. 2. The peak magnitude of the response to nerve stimulation was significantly increased in a concentration-dependent manner by ARL 67156 (5-100 microM) and the size of the neurogenic response at 4 Hz was approximately doubled in the presence of ARL 67156 (100 microM). 3. ARL 67156 (100 microM) has a rapid onset of action. The enhancing effect on neurogenic contractions was maximal after 10 min, was well maintained for at least 30 min and was rapidly reversed, with responses returning to control levels 10 min after washout. 4. The neurogenic contraction in the presence of prazosin (0.1 microM) was purely purinergic, as it was abolished by the P2-purinoceptor antagonist, PPADS (100 microM). ARL 67156 (100 microM) produced a similar degree of enhancement of neurogenic responses in the absence and presence of prazosin, supporting the view that the enhancing effects of ARL 67156 on neurogenic contractions result from potentiation of the action of ATP. 5. Exogenous ATP and alpha, beta-methyleneATP produced rapid transient contractions. Responses to ATP were increased in magnitude and duration in the presence of ARL 67156 (100 microM), whereas those to the stable analogue, alpha, beta-methylene ATP were not significantly affected. 6. Contractions to exogenous noradrenaline (10 microM) and KCl (40 mM) were significantly enhanced by ARL 67156 (100 microM), but this potentiation was abolished by PPADS (100 microM). Therefore, this effect of the ecto-ATPase inhibitor may be due to a build up of endogenous ATP, increasing the sensitivity of the smooth muscle to other agonists. 7. It is concluded that ARL 67156 potentiates the action of ATP, and that when ATP acts as a neurotransmitter its postjunctional actions are greatly attenuated by enzymatic degradation.