Bulbocapnine enhanced the effect of histamine on the isolated perfused circulation of the rabbit ear and the isolated guinea pig ileum but not on cat blood pressure. Pyridoxal phosphate was capable of antagonizing the potentiating effect of bulbocapnine to histamine responses on the isolated guinea pig ileum. An explanation of this effect of bulbocapnine is discussed in the light of the known inhibiting effect of this substance on DAO.