OBJECTIVE Endothelium-derived relaxing factor (or nitric oxide) is thought to play an important role in control of blood flow in umbilical blood vessels at midgestation compared with term. Previous studies suggest that histamine releases endothelium-derived relaxing factor from umbilical arteries. In this study we intended to clarify the mechanism by which histamine releases endothelium-derived relaxing factor and causes vasorelaxation in human umbilical artery at the midstage (18 to 22 weeks) of gestation. STUDY DESIGN By means of very thin muscle strips that allow rapid diffusional access of applied drugs (in a few seconds), contractile properties of human umbilical artery were examined. Isometric tensions were measured in response to potassium chloride (39 mmol/L) or caffeine and inhibitory effects of histamine, A23187, glyceryl trinitrate, and 8-bromo-cyclic guanosine monophosphate on these contractions were also examined. RESULTS Histamine (0.01 to 0.1 mumol/L) did not inhibit 39 mmol/L K(+)-induced contractions of tissues taken at the terminal (38 to 41 weeks) stage of gestation. However, at midgestation histamine (0.01 to 0.1 mumol/L), A23187 (10 mumol/L), and 8-bromo-cyclic guanosine monophosphate (membrane-permeable analog of cyclic guanosine monophosphate, 0.1 mmol/L) inhibited 39 mmol/L K(+)-induced contractions. The inhibitory effects of histamine were antagonized by mepyramine (an H1 antagonist), L-NG-nitro arginine, methylene blue, and Ca++ depletion of the extracellular space but not by cimetidine (an H2 antagonist). Caffeine produced contractions both in the presence and absence of extracellular Ca++ possibly because of the release of Ca++ from intracellular storage sites. Glyceryl trinitrate and 8-bromo-cyclic guanosine monophosphate reduced the caffeine-induced contractions in Ca(++)-free solution. In addition, 10 mumol/L cyclic guanosine monophosphate did not attenuate the Ca++ sensitivity for contractile elements. CONCLUSION These results suggest that (1) histamine coupled to the histamine H1 receptor increases intracellular Ca++ concentration to stimulate nitric oxide synthase in human umbilical endothelial cells, (2) nitric oxide from endothelial cells activates guanylate cyclase to produce cyclic guanosine monophosphate in the umbilical smooth muscle cells, and (3) cyclic guanosine monophosphate relaxes the umbilical tissues, perhaps as a result of the activation of a Ca++ extrusion system.