The effect of five sympathomimetic amines and some of their acetyl derivatives on the blood pressure of the rat was determined on the left carotid artery. After pretreatment with chlorisondamine (1 mg/kg subcutaneously) the blood pressure rise by sympathomimetic amines and their acetyl derivatives was compared with that of adrenaline. If the potency of adrenaline is specified as 100, the potencies of the other drugs are phenylephrine (metaoxedrinum, NFN) 37, tyramine 1.1, O-acetyltyramine 0.52, amphetamine 0.50, O-diacetylphenylephrine 0.25, ephedrine 0.23, O-acetylephedrine 0.02, N-acetylphenylephrine 0.01. The effects of N-acetyltyramine, N-acetylephedrine and N-acetylamphetamine are even weaker. Reserpine 5.0 or 0.05 mg/kg intraperitoneally 24 hours before the experiment increased the blood pressure rise by the directly acting sympathomimetic amines and their acetyl derivatives, but decreased the effects of the indirectly acting drugs. After treatment with phenoxybenzamine (2 mg/kg intraperitoneally), adrenaline exhibited the greatest blood pressure decrease and the effects of the other drugs in descending order: orciprenaline, O-acetyltyramine, phenylephrine, ephedrine, amphetamine, O-diacetylphenylephrine and O-acetylephedrine. Tyramine did not show any blood pressure decrease. The blood pressure decrease by sympathomimetic amines and by their acetyl derivatives was probably due to beta-receptor stimulation because it was prevented by propranolol. The N-acetyl derivatives recembled their parent drugs with regard to the immediate onset and short duration of their effects. The O-acetyl derivatives exhibited slower onset and longer duration of effect than their parent drugs. Physostigmine-pretreatment diminished the rise in blood pressure by O-acetyltyramine, but the effect of tyramine remained unchanged.