An attempt was made to quantitatively evaluate the effect of sympathomimetic amines by measuring the decrease in ear temperature, which reflects the constriction of the peripheral vessels, in intact rabbits and dogs. In rabbits, the decrease in ear temperature was proportional to the increasing dose of epinephrine, norepinephrine or ethylphenylephrine when the drugs were given intravenously. The linear regression between the logarithmic dose and the area under time-decreasing curve was highly significant for each drug. The action of l-epinephrine was approximately two times more effective than that of dl-norepinephrine, and the effect of ethylphenylephrine persisted longer than that of the others. Little decrease in ear temperature was observed after intravenous injection of isoproterenol. The ear temperature lowering action of norepinephrine and ethylphenylephrine was inhibited by the pretreatment with phenoxybenzamine. In rabbits and dogs, when ethylphenylephrine was orally given, the linear regression between the logarithmic dose and the area under time-decreasing curve was also highly significant. The present results indicate that the determination of ear temperature is useful for the evaluation of potencies of preparations containing the vasoconstrictors tested.