For the purposes of this study, sulfentanil is administered to conscious men in increasing doses (0,00015, 0,0003, 0,0006 mg/kg) by intra-muscular injection, and the results are compared with those obtained in the same condition by administration of morphine (0,100, 0,150, 0,200 mg/kg). The administration of both drugs provokes the same side-effects. The failure of the breathing rate is as fast as observed following the administration of morphine in equi-analgesic doses. As in the case of analgesia, the time taken for the falling off of the breathing rate to occur, and its duration are shorter with sulfentanil, than with morphine. In any case their persistence is too great for one, to be able to consider sulfentanil as a drug of very short-term effect. The effects of sulfentanil on the cardiac rhythm (bradycardia) and on arterial pressure (low blood pressure) are slightly less intense, but above all, much shorter than that of a equi-analgesic dose of morphine. The other side-effects which are clinically discernable, particularly those felt by the patient, vomiting and changes in behaviour patterns are minimal; certainly lesser after administration of S than after tha of M. The sedative value are the same and often identical after administration of both drugs. Finally, the euphoric effect of sulfentanil appears to be slighter than that of morphine.