2. Morphine and heroine were found to exert a selective depressant action on the central expiratory mechanism, and the slower rate, with relatively unaltered depth, seemed to be due at least partly to the slower rate of emptying the lungs. Codeine had no depressant action on the respiration of decerebrated cats.

3. Larger doses of morphine or heroine had no further depressant effect on rate or depth of breathing after expiration was made passive, unless circulatory depression appeared, and failure of circulation seemed to be the cause of respiratory depression, rather than the reverse relation. In decerebrated animals large doses of morphine and moderate doses of codeine stimulated the spinal cord, and expiration became active, with a faster rate of breathing. The characteristic action of morphine and heroine on the respiration of the cat is apparently limited to a depression of active expiration.