Histamine release was shown in the course of seven orthotopic homologous liver transplantations in the young Landrace pig. Elevated plasma histamine levels in the right atrium of the donor and recipient were found after introduction of anaesthesia, after exposure of the liver and its vessels, and following revascularization. The histamine release following revascularization is so massive that it can explain the hypotensive reactions observed in this phase of the operation. The increase of the plasma histamine concentration in the other phases of transplantation is so small that it is very probably without effect on the blood pressure. In the portal plasma, high plasma histamine levels were measured during the exposure of the liver, especially in the donor, but to a lesser degree also in the recipient. However, the liver completely took up this released histamine, and no increase of plasma histamine level could be demonstrated in the right atrium. Various possibilities exist to explain the development of hypotension following revascularization: anoxia, acidosis, elevated serum potassium levels, endotoxins, VDM, kinins, prostaglandins and the decrease of the circulating blood volume due to sequestration of blood in the transplanted liver. From the results of our experiments, however, it was concluded that histamine release plays an important role in the development of hypotension following revascularization.