Some data on drug effects in diseased human muscle, particularly in myotonic muscle, are reviewed. The effects of succinylcholine appear to be similar to those observed in denervated muscle. The finding of abnormal drug effects in known genetic disease has been recently supplemented with new observations of pharmacogenetic disease, that is, genetic defects which become apparent or fatal only through drug exposure. In patients with malignant hyperthermia the general anesthetics halothane and succinylcholine produce a dramatic rise of body temperature, usually accompanied by rigidity. Observations of the caffeine response of isolated muscle from such patients, and studies of the sarcoplasmic reticulum suggest that the intracellular calcium storage mechanism is subject to abnormal inhibition by anesthetics. A case of familial, acute ethanolinduced muscle destruction has been briefly described. It seems that there are still genetic defects of muscle to be discovered which permit normal function until drugs are present.--Kalow, W. Succinylcholine and malignant hyperthermia. Federation Proc. 31:1270–1275, 1972.