The effect on 5-hydroxytryptamine release from rabbit platelets to plasma of ten known sympathomimetic or anorectic phenethylamines and eight N- or O-acetyl derivatives of these substances were studied in order to find possible structure-action relationships and a possible correlation to the pulmonary hypertension inducing ability of some anorexigens. Among the mainly indirectly acting sympathomimetics amphetamine and ephedrine showed a similar high 5HT-releasing effect. Tyramine was slightly weaker, while the direct acting phenylephrine (metaoxedrine) and orciprenaline were much weaker. Acetylation increased the 5HT releasing effect of amphetamine, ephedrine and tyramine but decreased this effect of phenylephrine and orciprenaline. Among the anorectic phenethylamines the 5HT-releasing effect decreased in the following order, aminorex, amphetamine, clophorex, chlorphentermine, diethylpropion (amphepramon), phentermine. With regard to aminorex, which is a releaser of platelet 5HT and an inhibitor of both MAO activity and 5HT uptake in pulmonary tissue, it seems possible that the pulmonary hypertension can be caused by high concentration of free 5HT.