A comparison of monoamine oxidase activities in the hypothalamus and striatum between malignant hyperthermia-susceptible (Pietrain) and malignant hyperthermia-resistant (Landrace/Large White) pigs showed no significant difference between the two breeds. The concentrations of noradrenaline, dopamine and their non-O-methylated metabolites did not reflect the low activities of monoamine oxidase type A differentially. The malignant hyperthermia-susceptible pigs had significantly greater concentrations of 3,4-dihydroxyphenyl glycol in the striatum, and of 3,4-dihydroxymandelic acid in the hypothalamus. Consequently, in the brain, low monoamine oxidase type A activity does not appear to be involved in susceptibility to malignant hyperthermia. In addition, monoamine oxidase activities in the heart, liver, kidney and intestinal mucosa and catechol-O-methyl transferase activities in the kidney were the same in the susceptible and resistant pigs.