We examined whether the laryngeal mask could be used effectively in 70 patients breathings spontaneously during closed-circuit anesthesia. After administration of oxygen 101.min-1, anesthesia was induced and the laryngeal mask inserted. After 6 min of denitrogenation (3.51.min-1 nitrous oxide and 1.51.min-1 oxygen), the fresh-gas flow was decreased to the minimum required to maintain refilling of a ventilator bellows (Carden Ventmasta); nitrous oxide was turned off. A vaporiser outside the breathing system was set either by observing an end-tidal agent monitor or at 3-5 times greater than the required end-tidal concentration. The total fresh gas flow, hemodynamics and respiratory variables were monitored. Several types of operations were performed and lasted 9-126 min (mean 37 min). The average total fresh gas flow during the closed circuit period was 340 ml.min-1 (range 200-500 ml.min-1). Blood pressure, heart rate and respiration were stable throughout anesthesia and there were no major side effects attributable to the technique. We believe that closed-circuit anesthesia in patients breathing spontaneously through the laryngeal mask, as described here, can be used safely, conveniently and effectively. This technique is suitable for surgical operations of short duration.