The relatively short half life of verapamil necessitates divided daily dosing in the treatment of angina, hypertension and arrhythmia. To reduce dosing frequency and increase patient compliance and therapeutic efficacy, a controlled-release once-daily verapamil formulation (Verelan) has been developed in three dosage strengths, 120 mg, 240 mg and 360 mg. In order to investigate the dose linearity of this formulation in the 120 mg and 360 mg dose range, un unblinded, crossover, comparative evaluation of the three dosage strengths was performed in a population of 27 male volunteers. Each treatment period lasted nine days with a minimum of 7 days between periods. On Days 1 and 2 of each treatment period, the single dose phase was evaluated following administration of medication on Day 1 only with regular blood sampling over the 48 hour period. On Days 3 and 7 inclusive, the five-day steady phase was evaluated. Mean plasma profiles following administration of each dose demonstrated extended verapamil absorption up to 24 hours after dosing. In both the single dose and steady state phases a linear relationship was observed between increasing dose and pharmacokinetic response over the dose range of 120 mg and 360 mg. This linearity in response with increasing dose is in contrast to the non-linearity of verapamil's pharmacokinetics with conventional verapamil formulations previously described by an number of workers and may be due to a saturation of verapamil's hepatic first pass metabolic pathway.(ABSTRACT TRUNCATED AT 250 WORDS)