The effects of various salts on the activity of mitochondrial monoamine oxidase (MAO) from bovine liver were studied by measuring the initial and long-term enzyme reactions with an oxygen electrode and Warburg's manometer, respectively, using several monoamines as substrates. Salts with a -Cl- group, such as KCl and NaCl, or with a -NO3- group, such as KNO3 and NaNO3, increased the initial MAO activity with tyramine as substrate. NaCl strongly inhibited the long-term enzyme reaction. With tyramine as substrate, NaCl increased the activity only during the first 6 to 7 min of the reaction. Similarly with beta-phenylethylamine or butylamine as substrate, NaCl increased the initial MAO activity, but decreased the activity in the long-term reaction. On the other hand, with serotonin or benzylamine as substrate, NaCl decreased both the the initial reaction and the long-term reaction. The effect of NaCl on the enzyme with tyramine as substrate was reversible, while with serotonin as substrate it was irreversible. These results indicate that the effect of NaCl depends on the substrate used and suggest that bovine liver mitochondria contain at least two different forms of MAO.