Electrocardiographic abnormalities have been reported during ritodrine tocolysis. No previous studies, however, have included controls. The purpose of this study is to evaluate the electrocardiographic tracings of patients receiving ritodrine tocolysis and compare them with those of matched controls. Holter monitors were placed on eight patients receiving ritodrine tocolysis and eight control patients for 24 hours to make continuous electrocardiographic records. Twelve-lead electrocardiograms, serum glucose, and electrolyte concentrations were monitored serially in all patients. Four of the eight patients receiving ritodrine tocolysis demonstrated ST segment depression, while none of the control patients manifested any electrocardiographic abnormalities. The observed ST segment depression appears to be related to the degree of maternal tachycardia and the level of hypokalemia and hyperglycemia that occurs during early ritodrine tocolysis.