Plasma DBH and platelet MAO activities were measured by radioenzymatic assay in 10 Large-White and 20 Piétrain pigs 9 to 11 weeks old. Piétrain pigs within the same litter, challenged by halothane, were classified as malignant hyperthermia (MH) susceptible or not according to their reaction. The Large-White pig strain was not susceptible to MH. Plasma DBH activity did not differ according to strain or to MH susceptibility. Platelet MAO was lower in Large-White pigs but was not affected by MH susceptibility in Piétrain pigs. The results do not confirm any nervous differences in MH-susceptible pigs.