1. Forty hospitalized chronic schizophrenic patients were given a test of overinclusive thinking, and a vocabulary test, and then taken off all medication for a six-week period.

2. During this six-week period the behaviour of 11 patients became so disturbed that they had to be dropped from the experiment and put back on drugs.

3. At the end of this drug-free period, overinclusive thinking was re-assessed by two tests, and ratings of ward behaviour made independently by three nurses and their own psychiatrist.

4. The patients were then divided into two carefully matched groups. One was given Proketazine, a high potency phenothiazine drug, for six weeks. The other had placebo tablets. The experiment was "double-blind".

5. Following this treatment, the patients were given two more tests of overinclusive thinking, and their ward behaviour re-rated in the same way.

6. On maintenance doses of drugs, and after withdrawal from all drugs, these chronic schizophrenics were significantly less overinclusive than acute schizophrenics, and only very slightly more overinclusive than normal people in the drug-free state. Their overinclusive thinking was made slightly but not significantly worse by the withdrawal from maintenance doses of medication, on average, although there was considerable variability, some getting better and some worse.

7. `Proketazine' has a precisely similar effect, producing a very variable response, but no significant average improvement in those who had it.

8. The ward ratings of behaviour were not significantly changed following Proketazine treatment.

9. These findings suggest that:

(a) Contrary to expectation, overinclusive thinking does not seem to be responsible for the intellectual deterioration shown by a chronic schizophrenic group, since chronic schizophrenics, unlike many acute schizophrenics, are as a group scarcely more overinclusive than normals.

(b) Because they are not overinclusive in the first place, it is not surprising that these patients' overinclusive thinking as tested is not significantly influenced by drugs.

(c) Overinclusive thinking in acute schizophrenics may thus be a good prognostic sign, indicating that they are not likely to become chronic patients. However, it is essential to test this hypothesis, not by a cross-sectional study, but by a long-term follow-up study.

(d) As judged by their ward behaviour, chronic schizophrenics are relatively unaffected by a high potency phenothiazine drug (Proketazine). This is perhaps not surprising, as they are those patients who have remained in hospital in spite of the phenothiazine treatments they have received in the past.

10. It must be emphasized that this is not a completely representative sample of chronic schizophrenic patients, because of the need to eliminate 36 per cent. of the originally chosen group on account of disturbed behaviour.