Fentanyl is a strong, synthetic analgesic which may cause muscular rigidity when administered intravenously. To obtain a quantitative measure of the possible increase in muscle tone after intravenous fentanyl, the muscular tension of the right rectus abdominis was measured in 20 patients before and after administration of this drug. A traction transducer apparatus was fastened between the anterior and posterior rectus sheath in a right oblique laparotomy incision. Premedication was with pentobarbitone, and the anaesthesia and muscle relaxation were effected by thiopentone or enibomal and nitrous oxide-oxygen with 75% nitrous oxide, and suxamethonium infusion (0.2%) until the measurement of tension was started. Immediately after the action of suxamethonium had ceased, fentanyl, about 0.004 mg/kg body weight, was administered. An increase in tone was found in 15 cases (75%). The mean increase was 9.2 N. The influence of the anaesthesia upon the result is discussed, and it is concluded that fentanyl must be responsible for the increase in muscle tone.