A note on the distribution of [c]-histamine added to blood.

(14)C-Labelled histamine was incubated with canine or human blood at 37 degrees C. After 1 hr 80 to 100% of the added histamine could be recovered in unchanged form. When [(14)C]-histamine was added to whole blood in vitro it tended to become equally distributed between the cell and plasma fraction of the blood, and when the cell fraction of blood containing [(14)C]-histamine was suspended in [(14)C]-histamine-free plasma the labelled histamine tended to become equally distributed between cells and plasma. The pattern of distribution of intravenously injected [(14)C]-histamine in the blood of anaesthetized dogs seemed to be the same as that of histamine added in vitro. The injected histamine entered the cell fraction of the blood at a slow rate. These experiments indicate that in dog and man the blood cells are of little or no importance for the inactivation of histamine released into the circulating blood.