Despite a lack of clinical data in this area, conventional wisdom holds that morphine sulfate induces vagally mediated conduction defects, especially in patients with inferior myocardial infarction. To assess the accuracy of this "clinical pearl," the records of 244 patients admitted to the Barnes Hospital Cardiac Care Unit with suspected acute myocardial infarction were reviewed to determine the frequency of deleterious cardiovascular effects related to the administration of morphine sulfate. Of 184 patients (156 subsequently documented to have infarction) who received morphine sulfate, four patients had symptomatic hypotension temporally associated with morphine sulfate administration. This represented a frequency of 2.2 percent for all patients treated with morphine sulfate and a frequency of 2.6 percent in those with proved infarction. In each instance, the heart rate response was inappropriate, i.e., decreased or less markedly accelerated than might be expected given the reduced blood pressure, suggesting a vagal mechanism for the adverse effects. Only one of the four patients had inferior infarction, and in three of four instances, the adverse effect occurred after the first dose. All patients subsequently received morphine sulfate without evidence of toxicity. No case of narcotic-induced conduction abnormality was identified. This series, which is the most extensive evaluation of the topic, documents that adverse cardiovascular effects due to morphine sulfate are rare and do not conform to preconceived clinical doctrine. They consist of inappropriate heart rate responses to hypotension rather than conduction defects and are not particularly associated with inferior myocardial infarction.