Diisopropyl fluorophosphate (DFP) and other anticholinesterase (antiChE) agents have been found to induce marked hypothermic responses in laboratory rodents. To characterize the effects of DFP on autonomic and behavioral thermoregulation, rats of the Long-Evans strain were injected with DFP while housed in a temperature gradient. The gradient allowed for the measurement of selected ambient temperature T(sub a) and motor activity (MA) over a 6- to 7-day period. Core temperature T(sub c) and heart rate (HR) were also monitored simultaneously using radiotelemetry. Injection of the peanut oil vehicle led to transient elevations in T(sub c), HR, and MA, but no change in selected T(sub a). The next day animals were injected with 0.25, 1.0, or 1.5 mg/kg DFP. DFP (1.0 and 1.5 mg/kg) led to a marked reduction in T(sub c). The decrease in T(sub c) was accompanied by reductions in HR, MA, and selected T(sub a). During the first night after DFP, selected T(sub a) remained elevated as T(sub c) recovered to its preinjection level. The second 24-h period after 1.0 and 1.5 mg/kg DFP was associated with a significant elevation in the daytime T(sub c). In conclusion, with the option of using behavioral thermoregulatory responses, the hypothermic effects of acute DFP treatment are mediated by a selection for cooler T(sub a). An elevation in T(sub c) during recovery from acute DFP corroborates the many incidents of fever in humans exposed to anti-ChE agents.