The relative increase of heart work caused by aminorex in dogs is greater in the right ventricle than in the left. In contrast, noradrenalin, amphetamine, ephedrine and tyramine are more effective in the left ventricle. In contrast to sympathomimetics, aminorex increased the pulmonary vascular resistance more than the total peripheral resistance. This confirms that aminorex acts preferentially on the pulmonary circulation. None of the following substances can block the effect of aminorex on the pulmonary pressure of dogs: phentolamine, methysergide, promethazine, and a combination of phentolamine with methysergide or promethazine. As opposed to this, the increase in pulmonary pressure induced by noradrenalin, adrenalin, tyramine, 5-hydroxytryptamine or histamine can be blocked by phentolamine, methysergide, or promethazine. Phentolamine prevents the effect of aminorex on the pulmonary vascular resistance. The increase in the pulmonary artery pressure is caused by an increase in the cardiac output. Aminorex also acts preferentially on the pulmonary circulation of cats. This action is a peripheral one, because it is the same in spinal cats. Our experiments clearly show that the effect of aminorex on pulmonary circulation is not a sympathomimetic effect.