Inhibitory action of several compounds on rat liver monoamine oxidase was investigated according to Ferguson's rule and Mullins' hypothesis. Aliphatic primary alcohols, their acetates, aralkyl alcohols, phenethylamines, and piperidinoacetophenones decreased monoamine oxidase activity to about one-half of non-treated preparation at the concentration giving thermodynamic activities of 10^-1 to 10^-2. These results suggest that these compounds might be classified as drugs having a "physical toxicitiy" and affected monoamine oxidase by their physicochemical properties. Volume fractions of alcohols were also calculated and a theoretical curve calculated from δ_blo=11.5 and XV=6.7 fltted the experimental values.