CBA/J mice were injected intraperitoneally with the formamidine insecticide chlordimeform (CDM) while colonic temperature, preferred ambient temperature (Ta), and lethality were monitored. In the first experiment there was a dose-dependent decrease in colonic temperature when measured 60 min after administering CDM doses of 0, 15, 30, 60, and 75 mg/kg. The hypothermic effect of CDM was more pronounced at a Ta of 20 C than at 30 C. In the second experiment CDM at doses greater than 30 mg/kg caused a dose dependent reduction in preferred Ta from the normal of approx. 30 C down to approx. 22 C. Thus, the CDM-treated mouse lowers body temperature by selecting a cool Ta which accelerates the hypothermic effect. In the final experiment, a 90 mg/kg i.p. injection of CDM (the approximate LD50 dose) caused 10% and 0% mortality at a Ta of 20 and 30 C, respectively, and 80% mortality at a Ta of 35 C.