Isolated cat papillary muscles driven at a rate of 12 beats/min at 37.5 C were exposed tojeoncen-trations of cyclopropane, diethyl ether, Ethrane, halothanc, and methoxyflurane similar to those required to produce general anesthesia mvivo.Each anesthetic depressed peak developed ten-sion, maximal dp/dt, and the force-time integral of the twitch, and each shortened the time to peak tension. These variables were altered in qualitatively similar ways by all anesthetics tested, implying a common mode of action on the con-tractile process. When administered in cquieffec-tivc concentrations from the standpoint of producing general anesthesia (i.eat equal MAC's), the order of activity of the anesthetics in depressing contractility (from most to least depressant) was: Ethrane > halothanc > methoxyflurane > cyclopropane > diethyl ether.