Drug treatment of gastrointestinal diseases, which was previously limited to the use of antacids, anticholinergics, antispasmodics, cathartics and laxatives, has changed markedly over the past decade. Histamine H2-receptor antagonists (e.g. cimetidine, ranitidine and more recently famotidine and nizatidine) have revolutionised the treatment of peptic acid disorders, but their role is currently challenged by muscarinic-M1-receptor antagonists (e.g. pirenzepine), proton pump inhibitors (e.g. omeprazole), prostaglandin analogues and site-protective drugs (e.g. colloidal bismuth subcitrate and sucralfate). Newer antiemetics with prokinetic properties (e.g. metoclopramide, domperidone and cisapride) have also been introduced in the management of gastrointestinal motility disturbances, and new anti-inflammatory salicylates (e.g. olsalazine and mesalazine) have been developed for the treatment of chronic inflammatory bowel diseases. Finally, diphenoxylate and loperamide have gained wide clinical application as nonspecific antidiarrhoeal agents. The basic pharmacokinetic properties of the above agents are briefly reviewed with the main emphasis on the newer and more important drugs in current use. Furthermore, the effects of age and disease on pharmacokinetics, in addition to drug interaction potentials and pharmacokinetic-pharmacodynamic relationships, are discussed. The anti-inflammatory salicylates, nonspecific antidiarrhoeal agents, laxatives and cathartics will be dealt with in Part II.