The causes of nausea, vomiting and retching and how these are brought about are explained, and the present views on the mechanism of vomiting are discussed. The sites of action of sensory stimuli causing vomiting are linked in a figure with the probable sites of action of anti-emetic drugs. Three main groups of anti-emetic drugs are discussed, general sedatives and tranquillizers, hyoscine and atropine, and certain antihistamines. The results of some clinical trials are considered and suggestions made for the best combination of drugs. Undesirable side effects, particularly those of the phenothiazines, are stressed so that suitable precautions can be taken in their use.