Our objective was to identify biologic determinants of propranolol serum levels in 1308 patients after myocardial infarction (MI). Patients had had their MI within the previous month. A steady-state propranolol dosage of 40 mg every 8 hours produced a mean trough concentration of 42 ng/ml with extremely great (fiftyfold) interindividual variability. Univariate and multivariate analyses suggested that this variability was the result of many biologic factors. Serum levels were higher in women, in older patients, and in patients receiving concomitant therapy with other antiarrhythmic drugs. Serum levels were also higher in patients with elevated serum creatinine and lactate dehydrogenase levels. Serum levels were lower in black patients than in white patients. Also, serum levels in smokers were lower than those in nonsmokers, but only markedly so in the outpatient setting (6 months after the MI). The influence of sex and race on drug disposition has not previously been reported for beta-blocking drugs. Although a genetic deficiency in the oxidative metabolism of propranolol has been indicated, the frequency distribution of serum propranolol levels did not demonstrate a bimodal distribution for genetically distinct populations.