The influence of pregnancy-induced hypertension (pre-eclampsia) on muscarinic cholinergic receptors and on acetylcholinesterase (AChE) activity was investigated using frozen sections of the umbilical artery and vein. Pre-eclamptic patients undergoing Caesarean delivery and normotensive pregnant control woman undergoing Caesarean delivery with similar parity, gestation length and age were examined. Muscarinic cholinergic receptors were assayed in frozen sections of the umbilical artery and vein by a radioligand binding assay technique, using [3H]-N-methyl scopolamine (NMS) as a ligand. AChE was demonstrated with a histochemical technique associated with microdensitometry. [3H]-NMS was specifically bound to sections of both umbilical artery and vein in a manner consistent with the labelling of muscarinic cholinergic receptors. The affinity of the radioligand was similar in the two vessels, whereas the maximum density of binding sites (Bmax) was higher in the umbilical vein than in the artery. A faint AChE reactivity was observed in the tunica media of both umbilical artery and vein. In pre-eclampsia, a loss of [3H]-NMS binding sites not accompanied by changes in the affinity of radioligand was found. The decrease of muscarinic cholinergic receptors involved to a greater extent the umbilical artery than the vein. No differences in AChE activity were found at the level of umbilical artery and vein between control and pre-eclamptic subjects. These findings suggest that pre-eclampsia is characterized by a loss of muscarinic cholinergic receptors in the umbilical circulation not accompanied by changes of the acetylcholine catabolizing enzyme AChE. It is possible that the decreased density of vascular muscarinic cholinergic receptors in pregnancy-induced hypertension contribute to the increased resistance of the umbilical circulation occurring in pre-eclampsia.