MC4703 is a new phenothiazine derivative which contains a trifluoromethyl group and which was made available in the form of 10 mg. and 50 mg. tablets by E. R. Squibb & Sons for clinical evaluation as an ataractic or “tranquillizing” agent. The interesting results obtained with the drug justify the presentation of this brief, preliminary report, but a more detailed account of the investigation will be supplied in due course.