Abstract Although the literature on the effects of histamine is quite extensive it deals mostly with observations made upon dogs and cats, while observations upon reactions produced in the rabbit are particularly conspicuous by their absence. Possibly this is due to one of the peculiar anomalies of histamine in failing to produce an immediate depressor effect in rodents as it does in carnivora. This has been quite puzzling since histamine is known to be a strong stimulant of plain muscle, yet produces a pronounced fall in blood pressure of carnivora, apparently due to capillary dilation. In the rabbit the exact opposite reaction occurs—a rise in blood pressure, presumably due to vasoconstriction as one would expect. Dale and Richards (1) point out that vasodilator effects of histamine are purely peripheral effects upon the blood vessels, independent of any nervous connection. They conclude that the vasodilation is due to the relaxation of the tone of the capillaries since histamine has a constrictor effect upon the arteries. However, as Krogh (2) indicates they are not careful to differentiate sharply between capillaries and the smallest arteries, and he thinks what they noticed might be due to the action of arterioles beyond the range of the unaided vision, even though this is different from the action of larger arterial branches.