Hydrochlorofluorocarbons (HCFCs) are being developed as replacements for chlorofluorocarbons (CFCs) that deplete stratospheric ozone. The depletion of stratospheric ozone may increase the intensity of ultrviolet radiation at the earth's surface, which may be assodated with global, adverse human health effects. The greater tropospheric lability of HCFCs, which is due to the presence ofC-H bonds, reduces HCFC migration to the stratosphere; HCFCs should, therefore, cause less depletion of stratospheric ozone than CFCs. HCFCs under development include HCFC-22 (chlorodifluoromethane), HCFC-123 (2,2-dichloro-1,1,1-trifluoroethane), HCFC-132b (1,2-dichloro-1,1-difluoroethane), HCFC-134a (1,1,1,2-tetrafluoroethane), HCFC-141b (1,-dich l-fluoroethane, and HCFC-142b (1-chloro-1,1-duoroethane). With the exception ofHCFC-22, which is already in use, the metabolism and toxicity ofHCFCs have not been studied in detail. By analogy to chlorinated ethanes, predictions can be made about the possible metabbolism ofHCFCs, but there are insufficient data available to predict rates of metabolism. Although most HCFCs appear to show low acute toxicity, some HCFCs are mutagenic in the Ames test. Hence, future researh on HCFCs should include studies on the in vivo and in vibo metabolism of HCFCs as well as on their toxicity in in ivo and in vitro systems.