A method is described for the assay of circulating hormones after their injection or release into the circulation. The method is applicable to cats, dogs and rabbits, and consists of bathing or superfusing isolated smooth muscle preparations in a stream of heparinized arterial blood taken from and returned to the animal at a constant rate. The tone of the smooth muscle preparations was affected by small changes in the concentrations of various amines. Thus increases in blood concentrations of catechol amines can be assayed with the rat stomach strip and chick rectum preparations. The proportions of adrenaline and noradrenaline in a mixture can be determined. Circulating histamine can be assayed on the blood-bathed guinea-pig ileum and bradykinin on the rat duodenum preparations. The uses and limitations of the technique are discussed.