A controlled clinical comparison of fentanyl and morphine in 75 postsurgical patients is presented. In addition, the ability of the two drugs to produce respiratory depression was determined in six healthy volunteers. The results of these studies indicate that 0.2 mg. fentanyl intramuscularly is equianalgesic with 10 mg. morphine intramuscularly, and it would appear that there is no advantage in the use of fentanyl for the relief of postsurgical pain.