1. The blood pressure responses to sympathetic and parasympathetic stimulation of 100 patients suffering from the commoner types of mental illness were recorded before and after treatment. The responses of 10 healthy male subjects were recorded for control purposes. 2. With the exception of hysteria, all the types of mental illness investigated showed abnormal autonomic patterns in terms of blood-pressure response but the individuals in each group showed a pattern which appeared to be characteristic of that group. 3. The possible reasons for the inconsistency in the observations of previous workers in this line of investigation were discussed. 4. Following treatment the tests revealed a relationship between clinical improvement and altered patterns of response in that the greater the improvement the nearer did the patterns approach 'normality' as judged by those of the controls. 5. The possible psychophysiological mechanisms involved in this relationship were discussed. 6. Suggestions were made on the influence of the autonomic pattern upon behaviour
