1 Twelve young (ages 30-39 years) and twelve elderly (ages 65-83 years) hypertensives were administered diltiazem twice daily for 2 weeks at doses up to 240 mg day-1. 2 Plasma was analysed for diltiazem, N-monodesmethyldiltiazem, and desacetyldiltiazem concentrations after a single 10 min intravenous infusion of 21.8 mg diltiazem HCl on day 1 and after the morning oral dose of 120 mg diltiazem base on day 14. 3 N-monodesmethyldiltiazem accumulated to higher plasma concentrations than desacetyldiltiazem at steady state on day 14 in both age groups. 4 Prolongation of plasma diltiazem half-life occurred after 2 weeks of oral diltiazem therapy in both age groups. 5 There were no significant differences between the young and elderly with regard to half-life, area under the curve, and the peak and trough plasma concentrations of diltiazem, N-monodesmethyldiltiazem, and desacetyldiltiazem; systemic clearance and volume of distribution of diltiazem were also similar in both groups.