Dose-response study of chloroform carcinogenesis in the mouse and rat: status report.

Chloroform is being administered to male Osborne-Mendel rats and to female B6C3F1 mice at concentrations of 0 (negative control), 200, 400, 900 or 1800 ppm in the drinking water. Matched control groups of both species receive a volume of water identical to that consumed by the corresponding 1800 ppm groups. At this writing, the animals have completed 23 months on test. Negative control and CHCl3-treated rat groups have shown typical growth curves, with dose-related relative decrements in body weight evident throughout the study. Following decreases in CHCl3 groups during the first 8 weeks, rat water consumption values have continued to increase slowly, but persistent relative dose-related decrements are evident. No initial treatment-related decrements are evident. No initial treatment-related mortality was seen in the rats. Survival is 21, 41, 45, 76, 70 and 64% for the negative control, 200, 400, 900, 1800 ppm and matched control groups, respectively. Survival values for mice at three weeks were 99, 94, 74 and 76% for the 200, 400, 900 and 1800 ppm groups, respectively. Mortality was apparently related to markedly decreased fluid consumption among some of the treated mice. Subsequent mortality has been less than 15% for all mouse groups. Except for acclimation effects during the first 2-3 weeks, body weights for the treated mouse groups have been generally within 10% of negative control values. Tissue changes in decedents have been similar in treated and control groups, both in rats and mice. Terminal sacrifice and histologic evaluations will be initiated after completion of 24 months on test.