Products of lipid peroxidation in the cerebrospinal fluid are found in higher concentrations in patients receiving phenothiazines than in control subjects matched for age and sex. Especially high concentrations are found in patients experiencing ill-effects from phenothiazine therapy. Raised concentrations of cerebrospinal fluid "phenanthroline copper" in these patients suggest that metal-catalysed mechanisms may promote the lipid peroxidation. It is suggested that lipid-soluble antioxidants, such as vitamin E, may be useful for the prevention and treatment of phenothiazine side-effects.