1. The blood-bathed organ technique was used for the continuous assay of histamine in circulating blood.2. Longitudinal strips of cat or kitten terminal ileum detect concentrations of histamine of 2-5 ng/ml; these are below the threshold at which hypotension is induced in dogs.3. The cat or kitten terminal ileum also contract to bradykinin. However, the simultaneous use of a cat jejunum strip, which is selectively sensitive to bradykinin, will distinguish between histamine and bradykinin.4. The utility of the method for detecting and quantitating circulating histamine was exemplified by measuring: (a) its half-life in the circulation, (b) its disappearance during a passage through the vascular beds of lungs, liver and lower limbs, and (c) its release into the circulation by compound 48/80.