To assess the contribution of its metabolites to the antihypertensive effects of diltiazem, a previously established rabbit model has been used to compare the pharmacokinetics and haemodynamic effects of the drug with those of its major metabolites deacetyldiltiazem (M1) and deacetyl-N-monodemethyldiltiazem (M2). Diltiazem, M1 and M2 were administered separately to each animal (n = 5 or 6 per study group) as a single 5 mg kg(-1) intravenous dose. Blood samples, systolic and diastolic blood pressure (SBP and DBP) and heart rate were recorded for each rabbit up to 8 h, and urine samples were collected for 48 h post-dose. Plasma concentrations of diltiazem and its major metabolites were determined by HPLC. The results showed that systemic clearance (CL) and volume of distribution at steady state (Vdss) were smaller for diltiazem than for the metabolites. Diltiazem and the metabolites reduced both SBP and DBP, the effects of diltiazem being most potent. Their effects on heart rate were highly variable and not statistically different between treatment groups (P > 0.05). These results indicate that diltiazem is a more potent hypotensive agent than M1 or M2, possibly because of the higher plasma concentrations secondary to the smaller CL and Vdss of diltiazem compared with the metabolites. The effects of the metabolites might, however, be more sustained.