Comparative bioavailability of two formulations of diltiazem (Dilzene, CAS 42399-41-7), a calcium antagonist, was evaluated on 10 healthy volunteers (5 males and 5 females) in a cross-over study. A single dose of 120 mg of diltiazem was administered to the volunteers in the form of either two 60-mg tablets or a 120 mg controlled-release tablet. Plasma concentrations of diltiazem over a 24-h time interval were determined by HPLC analysis. Results of this investigation demonstrate that the controlled-release formulation of diltiazem has a lower Cmax value when compared to the 60 mg conventional tablet formulation, but a longer tmax and a superimposable AUC.