Halothane present in the ambient atmosphere was continuously measured in each of two operating rooms during the conduct of surgical anesthesia. Concentrations were determined on-line with a mass spectrometer and found to vary with sampling site, breathing system used, and the scavenging system employed to remove overflow anesthetic gases. Concentrations of halothane measured within a 3-foot radius of the anesthesia equipment averaged 8.7 ppm when a nonrebreathing circuit was used (flow 10 l/min), and 4.9 ppm with a semiclosed circle system (flow 4–5 l/min). End-tidal concentrations of halothane averaged 0.21 ppm in 81 samplings from operating room nurses and 0.46 ppm in 36 samplings from anesthetists. Residual concentrations were present in many individuals 16 hours after exposure. A significant reduction in atmospheric contamination of the operating room was obtained by use of appropriate scavenging equipment. The implications of these findings are discussed.