Hydrochlorofluorocarbon 123 (HCFC 123) is one of the chemicals being considered as a replacement for the chlorofluorocarbons. Four subchronic inhalation toxicity studies from 1 to 3 months in duration have been conducted with HCFC 123. One study utilized rats and dogs, while the others were limited to rats only. The exposure levels have ranged from 300 ppm up to 20,000 ppm. Although the studies were conducted over a 14-year period, the results were consistent. In all studies, increases in liver weights were seen at 1000 ppm and above; additionally, one showed this effect at 500 ppm. Histopathological findings were minimal, consisting primarily of focal necrosis in the liver of the dogs at 10,000 ppm. Induction of peroxisomal activity, lowering of serum cholesterol and triglyceride levels, and an increase in urinary fluoride levels were also seen. The 4-hr LC50 in the rat has been reported as 35,000 ppm. At 20,000 ppm for 6 hr, the total daily dose on a concentration times time basis is almost equal to the LC50, yet, in the 4-week study, with 20 exposures at this level, there was no mortality or even marked signs of toxicity. There appeared to be no evidence for cumulative toxicity from multiple exposures in these studies. Overall, HCFC 123 appears to have a low level of toxicity by the inhalation route.