The effect of serotonin, PGE2 and PGF2α on the vascular tension in human umbilical arteries has been investigated during in vitro perfusions. All autacoids induced dose-dependent biphasic pressure responses when added to the perfusate. When added to the extraumbilical fluid, serotonin induced monophasic vasoconstrictory responses, whereas PGE2 and PGF2α did not elicit significant changes except for minor pressure oscillations in some experiments. The results obtained suggest a contributory role of the endothelial layer in the action of these autacoids on the smooth muscle tension.