The purpose of this investigation was to compare the effects of halothane on malignant hyperthermia (MH) and normal isolated muscle bundle performance during isometric contraction and relaxation phases. Mechanical parameters were measured: peak tension (PT), time to peak tension (TPT) and positive peak of isometric tension derivative (+dP/dt max) characterized the contraction phase. Half-relaxation time (RT 1/2) and negative peak of isometric tension derivative (-dP/dt max) characterized the relaxation phase. The ratio R= (+dP/dt max) /(-dP/dt max) was used to study the coupling between contraction and relaxation under isometric condition. In normal muscle, halothane increased PT by nearly 40% without altering TPT. The +dP/dt max value increased concomitantly with the -dP/dt max values, thus no changes in R was observed. In MH muscle, PT was first potentiated (0.5-1.0 vol% halothane) and then depressed (2.0–3.0 vol% halothane). TPT and +dP/dt max were not altered whereas RT 1/2 increased progressively with concomitant decrease in -dP/dt max, thus R increased by nearly 40%. The amplitude of MH muscle contracture with stepwise concentrations of halothane was correlated with the increase of RT 1/2 and R, and the decrease of -dP/dt max. These results suggest that halothane alters the relaxation phase more than the contraction phase in MH human skeletal muscle compared to normal muscle.