1. Strips of human saphenous veins were superfused with Krebs-Henseleit solution at either 25 degrees C or 37 degrees C. Constrictor responses to electrical stimulation (10 Hz, 40 s) but not to exogenous noradrenaline (0.1, 1 microM) were abolished by guanethidine (10 microM) and tetrodotoxin (1 microM). Hence, responses to electrical stimulation are due to action potential-induced release of sympathetic neurotransmitters. 2. Constrictor responses to electrical stimulation and noradrenaline were reduced by the alpha 1-adrenoceptor antagonist, prazosin (0.3 microM) as well as by the alpha 2-adrenoceptor antagonist, rauwolscine (1 microM). The combination of prazosin and rauwolscine abolished constrictor responses to noradrenaline at 25 degrees C and 37 degrees C. However, constrictor responses to electrical stimulation were partly resistant to alpha-adrenoceptor blockade by prazosin and rauwolscine (at 25 degrees C about 30%). Residual constrictor responses to electrical stimulation were also observed in the presence of the combination of prazosin (3 microM) and rauwolscine (10 microM) as well as in the presence of phenoxybenzamine (10 microM). 3. Veins, incubated with [3H]-noradrenaline, released tritium upon electrical stimulation (10 Hz, 40 s). Moreover, electrical stimulation also induced an overflow of ATP amounting to 4.8 +/- 1.5 pmol g-1 at 25 degrees C and 2.0 +/- 0.5 pmol g-1 at 37 degrees C. Both tritium and ATP overflow were abolished by tetrodotoxin (0.5 microM). The combination of prazosin (0.3 microM) and rauwolscine (1 microM) increased tritium overflow at either 25 degrees C or 37 degrees C by about 120%, but reduced ATP overflow by about 70%. Hence, a significant percentage of the electrically evoked ATP overflow seems to be released from non-neuronal cells upon activation of alpha-adrenoceptors by endogenous noradrenaline. The remaining ATP overflow, which was resistant to alpha-adrenoceptor blockade, may reflect neuronally released ATP.4. ATP (300 MicroM) and alpha,Beta-methylene-ATP (1, 10 MicroM), both induced constrictor responses. The P2-purinoceptor antagonist, suramin (300 MicroM) markedly inhibited constrictor responses to ATP and alpha, beta-methylene-ATP, but not those to electrical stimulation and to noradrenaline. Moreover, suramin(300 MicroM) failed to diminish the alpha-adrenoceptor blockade-resistant constrictor response to 10 Hz.5. In conclusion, constrictor responses to sympathetic nerve stimulation in human saphenous veins are mainly but not exclusively mediated by neuronally released noradrenaline. There is a concomitant release of ATP and noradrenaline. P2-purinoceptors which mediate vasoconstriction are present; however,a role of neuronally released ATP in constrictor responses to electrical stimulation could not be established. Therefore, the nature of the sympathetic transmitter responsible for alpha-adrenoceptor blockade-resistant constrictor responses remains unknown.