1. Rats were studied in Hall's open-field test 1 1/2 or 3 hours after various doses of 10 psychotomimetic drugs. After 8 of these, changes in behaviour at the lowest effective dose used included significant decrease in emotional defaecation. This effect was also found after a ninth drug, harmine, at a higher dose but with dimethyltryptamine it did not occur at any dose level, though there was a trend in this direction and a dose-effect relationship was apparent. 2. Three stimulant and 5 depressant drugs were tested but no consistent pattern of activity emerged. The only drug which caused decrease in emotional defaecation, thiopropazate, also caused concurrent decrease in all other measured categories of behaviour. 3. The lowest doses of all drugs causing changes in open-field behaviour were 1 1/2 to 20 times higher than the accepted human doses except with LSD-25 where the ratio was 1:250-500 and reserpine where it was 1:100. 4. The potential usefulness of the test in the study of psychotropic drugs is discussed. © 1963 Springer-Verlag.