Amlodipine is a 'second generation', long-acting calcium antagonist. To characterise the binding properties of this drug saturation binding studies were undertaken using (-)[3H]amlodipine and rat cardiac membrane fragments. (-)[3H]Amlodipine bound to a single population of high affinity binding sites with a KD of 1.64 +/- 0.17 nM, a Bmax of 0.45 +/- 0.08 pmol/mg protein and a Hill coefficient approaching unity. Binding was slow and required up to 5 hours to reach asymptote during incubation at 25 degrees C. The specific binding was totally inhibited by (-)amlodipine and (-)D600 and partially inhibited by (+)PN200-110, Bay K8644, (+)D600 and d-cis diltiazem. These results indicate that (-)[3H]amlodipine interacts strongly with the phenylalkylamine as well as the dihydropyridine binding sites. It also interacts with the benzothiazepine binding sites. The inhibition of (-)[3H]amlodipine binding by D600 is stereospecific, (-) greater than (+)D600. Bound (-)[3H]amlodipine dissociated slowly from its binding sites, less than 40% dissociation occurring during 5 hours of incubation (k-1 = 1.53 x 10(-3) min-1). These results indicate that the binding profile of amlodipine differs from that of other dihydropyridine-based Ca2+ antagonists. In addition they explain its slow onset of action, and slowed recovery on withdrawal.