PURPOSE: Inhibition of phosphodiesterase III (PDE-III) by enoximone elevates cyclic AMP (cAMP) content and intracellular calcium in human cardiac muscle. An involvement of cAMP in the pathogenesis of malignant hyperthermia (MH) has been suggested, because a higher basal content of cAMP was found in skeletal muscles from MH susceptible (MHS) than in normal (MHN) individuals and swine. In this study the in vitro effects of enoximone on skeletal muscles and the possibility to distinguish MHS from MHN patients were investigated. METHODS: Muscle biopsies from 37 patients with clinical suspicion for MH were obtained. The patients were first classified as MHS, MHE (equivocal result) or MHN by the caffeine halothane contracture tests according to the procedure of the European MH Group (EMHG). MHE-patients and patients with neuromuscular diseases were excluded from the study. Enoximone was added cumulatively every five minutes to surplus muscle specimens to obtain organ bath concentrations of 0.2, 0.4, 0.6, 0.8, 1.2 and 1.6 mmol/l. The in vitro effects of enoximone on muscle contractures and twitch were measured. RESULTS: Twelve patients were classified as MHS and 21 as MHN by the EMHG criteria. Enoximone induced contractures in skeletal muscles from all patients. MHS muscles developed contractures at significantly lower bath concentrations of enoximone than MHN muscles. Contractures of MHS compared to MHN muscles were significantly larger at bath concentrations of 0.4, 0.6, 0.8 and 1.2 mmol/l enoximone. No overlap in maximum contractures was seen between MHS and MHN muscles at bath concentrations of 0.6 and 0.8 mmol/l enoximone. Muscle twitch increased in both groups after administration of enoximone. CONCLUSION: The PDE-III-inhibitor enoximone induces contracture development in skeletal muscles. These contractures were attained at lower concentrations and were larger in MHS compared to MHN muscles. In vitro diagnosis of MH by a contracture test with enoximone appears to be possible. Furthermore, regarding these results a trigger potency for MH by enoximone could not be proved, but the use of enoximone in MHS patients might be dangerous.