This study was tried to investigate sympathomimetic action of acetaldehyde in rabbits. In normal whole rabbits, the intravenous injecton of acetaldehyde (2 to 20㎎/㎏) produced a typical elevation of blood pressure. These responses were not abolished by atropine-treatment or ligation of both adrenal veins. Acetaldehyde-induced pressor action was potentiated by pretreatment with chlorisondamine and debrisoquine, but not by cocaine without decrease of its blood pressure. In rabbits treated with reserpine to deplete catecholamines, the sympathomimetic acetaldehyde elicited a fall in blood pressure whereas tyramine-induced pressor response was disappeared without any depressor responses. These experimental results suggest that the sympathomimetic acetaldehyde appears to exert its action partly by the release of catecholamines from tissue stores other than the adrenal medulla, and that the releasing action of acetaldehyde appears to differ from that of compound such as tyramine and phenylethylamine, and that there are species differences between rabbits and cats in sympathomimetic activity of acetaldehyde.