A review with 80 references, on the low mol. weight ligands, peptide ligands, non-peptide ligands, and microorganism-derived substances as agonists and antagonists for receptors with clin. interests. Motilide as the agonist for motiline receptor, devazepide as the antagonist for cholecystokinin A receptor, and DuP753 and TCV-116 as the antagonists for angiotensin II receptor are discussed as low mol. weight non-peptide agonists and antagonists. IJ2702-I and II as the antagonists for muscarinic acetylcholine receptor, Sch42029 as the antagonist for dopamine receptor, ES-242-1 to 8 as the antagonists for NMDA receptor, WF11605 as the antagonist for leukotriene B4 receptor, and FR49175, FR900425, and phomatins as the antagonists for platelet activating factor receptor are discussed as microorganism-derived materials. Cholecystokinin receptor antagonists, endothelin receptor antagonists, WS9326 as the antagonist for substance P receptor, anantin and HS-142-1 as the antagonists for atrial natriuretic peptide receptor, hapalinodlinons A and B as the antagonists for arginine-vasopressin receptor, oxytocin receptor antagonists, and L-156602 (PD124966) as the antagonist for complement C5a receptor are discussed as peptide ligand receptor antagonists.