1 Spirally cut strips of the human saphenous vein and pulmonary artery were used to determine the pharmacological properties of the presynaptic prostanoid receptors involved in the modulation of sympathetic [3H]-noradrenaline release. Strips preincubated with [3H]-noradenaline were superfused with physiological salt solution containing inhibitors of uptake, and uptake2 and rauwolscine to eliminate involvement of presynaptic M2-adrenoceptors. Tritium overflow was evoked by transmural electrical stimulation (standard frequency: 2 Hz).2 In the saphenous vein, prostaglandin E2 (PGE2) inhibited the electrically-evoked tritium overflow; at the highest concentration investigated, tritium overflow was inhibited by more than 75% and the pECO value was 7.00. These effects were mimicked by prostaglandin El, the EPI/EP3 receptor agonist, sulprostone and the EP2/EP3 receptor agonist, misoprostol with the rank order (pECm): sulprostone (8.60) > PGE1 (7.25) > misoprostol (6.96). This rank order of potency suggests that the inhibitory effect of the drugs is mediated by presynaptic EP3-receptors. In contrast, PGF2, did not inhibit evoked tritium overflow; the IP/EP1 receptor agonist iloprost and the stable thromboxane A2 analogue U 46619 (9, 11-dideoxy-lla,9a-epoxy-methanoprostaglandin F2.) produced inhibition only at concentrations above 1 JiM. 3 The EP1-receptor antagonist, AH 6809 (6-isopropoxy-9-oxoxanthene-2-carboxylic acid) had no effect on the evoked tritium overflow nor did it modify the inhibitory effect of PGE2, further excluding involvement of inhibitory presynaptic EPI-receptors.4 PGD2 caused a facilitation of evoked tritium overflow in the saphenous vein; this facilitation is probably mediated by presynaptic DP-receptors, since it was abolished by the selective DP-receptor antagonist, BW A868C (3-benzyl-5-(6-carboxyhexyl)-1-(2-cyclohexyl-2-hydroxyethylamino)hydantoin). 5In the pulmonary artery, sulprostone (pECm value 8.35), misoprostol (7.70) and PGE2 (6.80) inhibited electrically-evoked tritium overflow. This rank order of potency is consistent with the involvement of inhibitory presynaptic EP3-receptors. 6 These results suggest that the sympathetic nerve fibres of both human saphenous vein and pulmonary artery are endowed with presynaptic inhibitory EP3 receptors. The EP3-receptors do not interact with the a2-autoreceptors. In addition, the human saphenous vein seems to be endowed with presynaptic facilitatory DP-receptors.