The amounts of cyclopropane, diethyl ether, halothane and methoxyflurane absorbed by rubber were determined by measuring the rates of infusion of anesthetic liquid or gas required to maintain a constant circuit concentration within a closed breathing system. Rates of absorption were directly proportional to the circuit concentration, the rubber-gas partition coefficients, the areas of exposed rubber, and the square roots of the diffusion coefficients. The observed uptake of each anesthetic was identical to that calculated from diffusion equations. The calculated diffusion coefficients of the above agents in rubber were 14, 1.7, 2.3, and 2.64 ± 10-5cmz/min, respectively. At equipotcnt anesthetic concentrations, the absolute amounts of anesthetic vapor absorbed by the breathing circuit are nearly the same for all agents. Absorption and elimination of these agents arc linear functions of the square root of time and cannot be accelerated by rapid flushing of the circuit. As a consequence, as much as 300 ml of anesthetic vapor may be routinely transferred from one case to a subsequent patient when agents are switched between cases.