1 The effect of 5-hydroxytryptamine on contractile responses to sympathetic nerve stimulation has been studied in the dog isolated saphenous vein.2 Electrical stimulation (0.1 to 10 Hz) of dog saphenous vein strips produced frequency-dependent contractions. Contractions produced by stimulation at 2 Hz were almost completely blocked by tetrodotoxin (3.1 x 10(-8) mol/l) or phentolamine (5.0 x 10(-6) mol/l) but mecamylamine (5.0 x 10(-6) mol/l) had little effect. This suggests that the contractions were mediated predominantly through noradrenaline release from postganglionic noradrenergic nerves.3 Contractions produced by intermittent electrical stimulation at 2 Hz were inhibited by 5-hydroxy-tryptamine (1.0 x 10(-9) to 1.0 x 10(-7) mol/l) in a concentration-dependent manner whilst contractions induced by exogenous noradrenaline were not affected.4 The inhibitory action of 5-hydroxytryptamine was most marked at low frequencies of stimulation and with low pulse numbers.5 High external calcium concentrations (3.9 and 5.2 x 10(-3) mol/l) reduced the inhibitory action of 5-hydroxytryptamine.6 Cyproheptadine (1.0 x 10(-8) mol/l to 1.0 x 10(-6) mol/l) or morphine (1.0 x 10(-7) mol/l to 1.0 x 10(-5) mol/l) did not antagonize the inhibitory action of 5-hydroxytryptamine. Methysergide (1.0 x 10(-7) mol/l) slightly reduced the contractions produced by electrical stimulation and only weakly antagonized the action of 5-hydroxytryptamine.7 It is suggested that a 5-hydroxytryptamine receptor exists presynaptically in the dog isolated saphenous vein strip and that stimulation of this receptor by low concentrations of 5-hydroxytryptamine inhibits the release of noradrenaline from noradrenergic nerves. This receptor type is resistant to blockade by ;classical' 5-hydroxytryptamine antagonists.