The in vitro contracture test determines the sensitivity of freshly obtained skeletal muscle specimens to caffeine or halothane applied to a bathing solution. Muscles from persons susceptible to malignant hyperthermia have lower contracture thresholds for these agents than do normal muscle. Thus, known concentrations of these agents must be accurately administered. We describe a system in which the bath concentrations of halothane (0 to 3%) are reproducible within 0.2% halothane from month to month. With use of this system, which includes an on-line gas analyzer, the measured halothane concentration within the bathing solution was independent of the performance of the vaporizer. The system is inexpensive and stable. The methods used to determine the bath concentrations of halothane and caffeine are reviewed. Finally, experimental equipment and a new recording system are described.