Diclofenac sodium (Voltaren) is a non-steroid anti-inflammatory agent of a new chemical structure, which is animal experiments shows a high degree of anti inflammatory, analgesic, and antipyretic activity in various pharmacological models. It inhibits prostaglandin biosynthesis in vitro and in vivo, and this inhibitory effect at least partly explains the mechanism of action of the preparation. In animal experiments diclofenac sodium is characterised by a broad therapeutic range. Also, its gastrointestinal tolerability is better than that of other highly effective non-steroid anti-inflammatory agents. Two of the metabolites produced during the biotransformation of diclofenac sodium in man are also biologically active. The activity of these two metabolites, however, is very much weaker than that of unchanged diclofenac sodium and is comparable to that of phenylbutazone.