Radiocarbon-labeled trichlorofluoromethane (FC-11; 14CC13F) and dichlorodifluormethane (FC-12; 14CC12F2) were separately inhaled by a female subject and a male subject. A predetermined volume of fluorocarbon (1000 ppm; 100 muCi) in air was delivered through a nonrebreating system and a tight-fitting face mask for 7-17 minutes. Total expired gases were collected during fluorocarbon exposure and afterward until no radioactivity was detectable. Expired 14CO2 and 14C-fluorocarbon were assayed. Urine was collected for 72 hours and assayed for nonvolatile radioactivity. Total recoveries of FC-11 were 99.5 and 79.4 per cent in the woman and the man, respectively. Total recoveries of FC-12 were 95.4 and 103.2 per cent. Traces of radioactivity were found in urine (FC-11, 0.07 and 0.09 per cent; FC-12, 0.02 and 0.03 per cent) and in exhaled carbon dioxide (FC-11, 0.13 and 0.10 per cent; FC-12, 0.08 per cent in both subjects). Total metabolites were equal to or less than 0.2 per cent of the administered dose. The amount of radioactivity in urine was insufficient to permit identification of possible fluorocarbon metabolites. The trace of metabolites could be products of radiolabeled impurities. (Key words: Gases, non-anesthetic, dichlorodifluoromethane (Freon 12); Gases, non-anesthetic, trichlorofluoromethane (Freon 11); Pharmacology, fluorocarbons.)