The purpose of this study was to determine the relative bioavailability of Cardizem CD compared to Tiazac after single and multiple doses. Twenty-three healthy males were enrolled in this open-label, two-way, complete crossover investigation. During each of the two treatment periods, a single 240-mg dose of diltiazem HCl was given in the morning on study day 1, then once daily on days 3 through 9. Serial plasma samples were obtained and pharmacokinetic parameters were calculated from the single-dose and steady-state concentration-time profiles. After single doses, mean diltiazem maximum plasma concentration (Cmax ) was 46% higher with the Tiazac formulation compared with Cardizem CD, and the mean area under the plasma concentration-time profile (AUC) was 19% higher with Tiazac. At steady-state, similar Cmax and AUC for the 24-hour dosing interval were found for Cardizem CD and Tiazac. However, Tiazac produced a 21% lower diltiazem minimum plasma concentration, a 28% lower trough concentration (the concentration in the plasma sample obtained just before the daily dose was given), and a 1.5-times higher fluctuation in maximum to minimum diltiazem plasma concentration compared with Cardizem CD. The pharmacokinetic profiles of the two pharmacologically active diltiazem metabolites, desacetyldiltiazem and N-desmethyldiltiazem, followed that of parent drug after single and multiple doses of Cardizem CD and Tiazac. From these results, it is concluded that the pharmacokinetic profiles of Tiazac and Cardizem CD are significantly different.