AbstractEhinger, B., G. Gennser, Ch. Owman, H. Perssonand N.‐O. Sjöberg.Histochemical and pharmacological studies on amine mechanisms in the umbilical cord, umbilical vein, and ductus venosus of the human fetus. Acta physiol. scand. 1968.72. 15–24.The responsein vitroof the umbilical vein and the ductus venosus to certain pharmacologically active drugs hls been correlated with the adrenergic and cholinergic innervation of this venous system in human fetuses of 20–24 weeks gestational age. No histochemically demonstrable adrenergic or cholinergic nerves occurred in the umbilical cord. In the intra‐abdominal part of the umbilical vein an increasing amount of adrenergic nerves were present in direction towards the ductus venosus. A distinct accumulation of adrenergic nerves was observed at the origin of the ductus venosus, concomitant with an increased thickness of the smooth muscle wall of the vessel. Few, if any, acetylcholinesterase‐containing fibres could be demonstrated in relation to the umbilical vein or ductus venosus. Noradrenaline, acetylcholine, and 5‐hydroxy‐tryptamine produced a contractile response in the intra‐abdominal portion of the umbilical vein as well as ir the first part of the ductus venosus. The noradrenaline response could be reproduced by tyramine and abolished by phenoxybenzamine, which potentiated the contraction induced by 5‐hydroxytryptamine. Atropine inhibited the effect of acetylcholine, but not that of noradrenaline. The presence of aminergic nerves is not indispensable for the existence of vascular smooth muscle receptors for the amines. The accumulation of adrenergic nerves and smooth muscle cells in the wall at the origin of the ductus venosus supports the theory of an adrenergic sphincler mechani