One hundred consecutive reports of reactions to intravenous anaesthetics Althesin, thiopentone and Epontol are reviewed and analysed. Ten reactions are attributed to causes other than the anaesthetic drug, and four are believed to have been caused by the muscle relaxant employed. The remaining 86 reactions were grouped according to their clinical presentation: histaminoid reactions ( 19), histaminoid with bronchospasm (33), bronchospasm (12), cardiovascular collapse (uu), delayed histaminoid reactions (6), and clonic contractions (5). None of the first four reaction types was associated with only one anaesthetic. A knowledge of the sales of Althesin has allowed the incidence of reactions to be estimated as between one in 11,000 and one in 19,000.