We examined the acute in vitro effects of cocaine on cell membrane potentials and contractility of 12-16 week old human foetal heart, to better assess the potential for the induction of serious arrhythmia, in utero, by this abused substance. Ventricular preparations were maintained in a tissue bath, and continuously provided with oxygen and glucose during the measurement of membrane potentials with microelectrodes, and developed force of contractions with microforce transducers. Cocaine (600 ng/ml) had a significant effect on the ability of the heart to produce action potentials of normal rising velocity, amplitude, and duration. Within 90 min., all electromechanical activity had ceased. Under the conditions of our study, the effects of cocaine were reversible, however, reversibility in vitro may have no counterpart in utero, and irreversible loss of cardiac function may result.