Abstract: The inhibitory activity of ephedrine on the amineoxidase of rat liver was determined in vitro and in vivo. The minimal inhibitory concentration of ephedrine in vitro was found to be 20 mM and this inhibitory activity of ephedrine on this enzyme was about one twentieth of that of iproniazid. In vivo, however, ephedrine did not produce any effect on the amineoxidase either in a single dose of 4 mg/kg or in an accumulated dose of 20 mg/kg given in 5 equal fractions at 30 min intervals.