The great number of PAF antagonists reviewed in this article clearly shows the tremendous effort made in the last 20 yr to explore the complex biological background of the mechanism of action of PAF and the potential clinical benefit of its antagonists. It is obvious that now highly potent, long-acting, and perorally applicable PAF-receptor antagonists are developed, and available for preclinical and clinical research. Another important fact is that the progress in the biochemical research designed to explore the structure of PAF receptors is impressive. This work has been expected to answer numerous questions concerning the tissue specificity of several PAF antagonists. For example, among the synthetic hetrazepine PAF-receptor antagonists developed by the Beaufour-IPSEN group, BN 50727 exhibits higher efficacy in the gastrointestinal tract (67) than in other tissues; alternatively, BN 50730 exerts more potent effect in the lung (66) than in other organs. At the present state of knowledge, the interpretation of these differences is unclear. Better knowledge about the PAF-receptor structure in the gastrointestinal tract may shed more light on the background of drug-receptor interactions responsible for tissue specificity. This may lead to more successful clinical development of PAF receptor antagonists in the near future.