Thirteen of 31 Belgian Landrace pigs developed malignant hyperthermia (MH) after breathing halothane. A short period of exercise 1 h before the administration of the triggering agent increased the incidence of the syndrome to 100% in eight similar pigs. Clinical symptoms were more marked and developed more rapidly in the exercised pigs. All the reacting pigs became typically acidotic, developed rigor and died. Serum Na+, K+, Ca2+, c.p.k., l.d.h. and protein concentrations were increased to a variable extent during the reaction and there was an increase in p.c.v. also. No hyperglycaemia was detected in pigs which were rested before receiving halothane. Four of the eight exercised pigs became markedly hyperglycaemic and plasma noradrenaline increased to higher values. Phosphocreatine and ATP decreased to low values and lactate increased in the muscles of all pigs which reacted. At the time of death, muscle glycogen had decreased significantly in the rested, but not in the exercised, MH pigs.