In order to assess the impact of anesthetic metabolism on elimination of halothane and enflurane, 3 groups of healthy, lean young men were given enflurane and halothane concomitantly for 2 h. Six awake volunteers (Group I) breathed 0.21% enflurane and 0.11% halothane in air. Four surgical patients (Group II) breathed the same anesthetic concentrations with 70% N2O. Five more surgical patients (Group III) breathed these volatile agents in concentrations that were 4 times greater, also in 70% N2O. All surgical patients received anesthetic adjuvant drugs (morphine, diazepam, thiopental and pancuronium) as clinically indicated. End-tidal and inspired anesthetic concentrations were measured during anesthetic administration and for 3-5 days thereafter. Anesthetic uptake, elimination and metabolism were calculated. Data from Groups I and II were comparable. Forty-five per cent of the halothane and 90% of the enflurane taken up were recovered. The smaller recovery of halothane was associated with a more rapid reduction in the end-tidal concentration of halothane than enflurane at the end of anesthesia. Group III differed from Groups I and II in that significantly more halothane (59%) was recovered, while 90% of enflurane was again recovered. For the 1st 30-205 min of elimination, the halothane end-tidal concentration decreased more slowly than did the enflurane end-tidal concentration. Evidently, a greater fraction of the halothane taken up at subanesthetizing, as oppposed to anesthetizing, concentrations is metabolized. This difference explains why elimination of halothane at subanesthetizing concentrations is more rapid than is elimination of enflurane, a less soluble but also less easily metabolized agent. Indeed, the absolute or total amount of drug metabolized was greater in the high-concentration study (Group III), only the relative percentage of drug taken up that was not recovered (a measure of the percentage metabolized) was smaller. N2O and the anesthetic adjuvants morphine, diazepam, thiopental and pancuronium had no significant effect on the extent of halothane or enflurane metabolism.