Plasma diltiazem concentration was determined for 24 hours after oral administration of 30, 60, 90 and 120 mg (sustained release tablets) in healthy adult white men. The plasma concentration was too low after the 30 mg dose to calculate pharmacokinetic variables. After administration of 60 mg (n = 12), 90 mg (n = 10), and 120 mg (n = 4), peak plasma concentrations were 72, 117, and 152 ng/cm3 and time to peak concentrations were 3.9, 3.3, and 4.0 hours, respectively. Half-lives for clearance from the plasma were 4.1, 5.1, and 5.6 hours and areas under the concentration-time curve were 514, 984, and 1258 ng/hour per cc, respectively. There was wide variability among patients after the administration of a single dose. The area under the curve also tended to increase more than the multiple of the dose administered. If the plasma diltiazem concentration is quantitatively related to efficacy and toxicity, then these data suggest that dosage requirements may vary considerably from patient to patient. Therefore, if a patient fails to respond sufficiently, the plasma drug concentration should be determined to see if adequate concentration has been attained. Alternatively, if the drug should prove to have a high therapeutic index, one might simply administer more than the usually required dose of diltiazem.