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In a non-double-blind, prospective, randomized study, the intra-operative electrocardiograms of 128 mildly hypertensive surgical patients were examined in order to determine the incidence of myocardial ischemia during anesthesia. No patient had been receiving chronic antihypertensive therapy prior to the study, but a single small oral dose of a beta-adrenergic blocking agent (labetalol, atenolol, or oxprenolol) was given to 89 of them along with premedication. Forty-four per cent of the untreated control patients and 61% of the patients pretreated with a beta-adrenergic blocking agent had normal preoperative electrocardiograms and no risk factors for coronary artery disease other than hypertension (this difference between groups was not statistically significant). During tracheal intubation and/or emergence from anesthesia, a brief, self-limited episode of myocardial ischemia was detected in 11 of 39 untreated control patients, and in two of 89 patients pretreated with a beta-adrenergic blocking agent (P less than 0.001). Tachycardia always accompanied the ischemic events, but a conspicuous increase in blood pressure did not. The authors conclude that mild hypertension, when untreated prior to the induction of anesthesia, is associated with a high incidence of myocardial ischemia; and that a single small oral dose of a beta-adrenergic blocking agent, given with pre-medication, can significantly reduce that risk.