The purpose of this communication is to present a group of patients who, while receiving treatment with one of the phenothiazine derivatives in average range of doses regarded as suitable for adults, developed a syndrome comprising mutism and a number of other neurological features. In the majority of these patients speech loss is almost complete; but, unlike aphasics, they may respond to repeated questioning by simple utterances usually spoken in a whisper, or else are capable on rare occasions, quite unexpectedly, of delivering remarks which are appropriate and may be spoken normally. Speech deprivation is as a rule associated with an inability to communicate either by gestures or writing and also an inability to perform many customary tasks. Mutism is, therefore, not an isolated disability, but is associated with a wide range of disturbances of behaviour and of level of consciousness. In addition, some of the patients exhibit one or more of the neurological toxic phenomena encountered in the course of medication with phenothiazine derivatives, e.g. akathisia, buccal dyskinesias, dystonias, tremor and parkinsonism.