The objective of this open-label, outpatient, parallel-group investigation was to compare the single-dose and steady-state pharmacokinetic profiles of young (n = 12; x = 41 ± 6 years) and elderly (n = 12; X = 69 ± 4 years) hypertensive patients following administration of a once-daily formulation of diltiazem. The study was comprised of two phases. The first phase was a lead-in phase used to establish hypertensive status. In the second phase, patients were administered a single- and multiple-daily dose regimen of 240 mg of diltiazem SR. Plasma samples were obtained at selected times and analyzed for diltiazem using a specific and sensitive HPLC assay. Biopharmaceutic parameter estimates were determined and analyzed for statistical differences. Qualitatively similar concentration-time profiles were observed between groups, suggesting similar release characteristics of the sustained-release formulation. However, significantly higher overall concentrations of diltiazem were observed following single-dose administration in the elderly, possibly as a result of an age-related decrease in the apparent oral clearance of diltiazem. A further reduction in the apparent oral clearance of diltiazem with multiple-dose administration was observed in the elderly resulting in even higher than projected concentrations of diltiazem. Thus, greater oral systemic availability of diltiazem in the elderly hypertensive patients may warrant closer clinical monitoring and possibly a reduction in dosage.