Abstract To determine if anaesthetic agents alter monoamine inactivation, we exposed tissue homogenates (liver, kidney and brain) from mice and rabbits to ether and chloroform vapors and carbon dioxide gas. These anaesthetic agents inhibited monoamine oxidase (MAO) activity against tryptamine and serotonin. Concentrations of anaesthetic agents that are achieved in the plasma of man during general anaesthesia caused a 27% (ether) and 49% (chloroform) reduction in mouse liver MAO; higher concentrations caused a 95% inhibition mouse or rabbit liver MAO. Kinetic analysis with tryptamine as substrate indicate that ether and chloroform are noncompetitive, reversible MAO inhibitors that preferentially inhibit Type B MAO. Ether and chloroform cause noncompetitive inhibition of serotonin oxidation by mouse liver MAO and competitive inhibition of serotonin oxidation by mouse brain and kidney MAO. Ether or chloroform did not alter catechol-O-methyltransferase activity from tissues of mice. Isolated blood platelets (rabbit and human) were used as a model system for neuronal uptake. Ether caused an irreversible inhibition of serotonin uptake by platelets.