We found that both variable bypass (Tec-type) and measured flow (Kettle type) vaporizers that nominally are turned off may permit anesthetic to leak into the bypass or the diluent stream. The contamination ranges from 1 to 3,300 parts per million of halothane (0.0001 to 0.33 percent). It appears to result either from diffusion of agent from the liquid reservoir into the bypass or diluent stream or from a reservoir in the piping connecting vaporizer to the diluent stream.