We intended to clarify whether (1) the amount of endothelium-derived vasorelaxing factor or nitric oxide released by histamine is sufficient to cause relaxation in human umbilical artery at different stages of gestation and (2) the functional role of endothelium-derived relaxing factor changes with the progress of gestation.By means of a bioassay cascade technique with very thin muscle strips, which allows rapid diffusional access of applied drugs (of the order of a few seconds), contractile properties were examined.At 18 to 22 weeks of gestation histamine produced only a minor contraction, but as gestation progressed the contractile responses increased. L-NG-nitro-arginine greatly enhanced the histamine-induced contractions. Histamine produced a concentration-dependent relaxation during the maintained contraction induced by 39 mmol/L K+ in tissues from 18 to 22 weeks or 30 to 32 weeks but not 38 to 41 weeks of gestation. Thus the concentration-relaxation relationship for histamine showed decreased sensitivity during gestation. The histamine-induced relaxation was enhanced by superoxide dismutase and completely blocked by L-NG-nitro-arginine or mepyramine (H1 antagonist). The concentration-relaxation relationship for the action of glycerol trinitrate relaxation decreased as gestation progressed. In a bioassay cascade an endothelium-intact umbilical artery from 18 to 22 or 38 to 41 weeks of gestation was used as a "donor" and L-NG-nitro-arginine-treated umbilical artery from 18 to 22 and 38 to 41 weeks as a "detector." In the presence of histamine the perfusate that had passed through a donor from 18 to 22 weeks (but not 38 to 41 weeks) attenuated the high K(+)-induced contraction in the detector only from 18 to 22 weeks.These results suggest that the amount of endothelium-derived relaxing factor and the sensitivity of smooth muscle to endothelium-derived relaxing factor decreased with the progress of gestation.