Binding characteristics of astemizole were studied in vitro in various receptor binding models and in vivo by determining the occupancy of histamine H1 receptors in guinea pig lung and cerebellum. In vitro, astemizole was found to have a high affinity for histamine H1 receptors, but great difficulties were encountered in proving this because of its high affinity for nonspecific binding sites. Since the equilibrium conditions were not reached in vitro, the real affinity of astemizole remains unclear and its receptor profile must be interpreted with caution. Nevertheless, the drug is certainly much more potent on histamine H1 receptors than on serotonin S2 and adrenergic alpha 1-receptors. Moreover, it was found to be devoid of antimuscarinic and antidopaminergic properties. The most striking property of this drug is its extremely slow dissociation rate from H1 receptors when assayed in vitro using [3H]-pyrilamine. Ex vivo experiments were performed in guinea pigs; astemizole was given orally to the animals, and the occupancy of H1 receptors in the lung and the cerebellum was determined in vitro by the [3H]-pyrilamine binding assay. Astemizole was found to occupy H1 receptors in lung at very low doses. Here again the most striking receptor binding property was its very long duration. The occupancy of H1 receptors in lung began to decline only 4-6 days after administration of the drug. However, there was a marked difference between the occupancy of peripheral and central receptors; indeed, in contrast to pyrilamine, astemizole at pharmacological doses did not reach the H1 receptors in the cerebellum, presumably because the drug does not readily cross the blood-brain barrier.