In the first experiment, mice were placed in a temperature gradient and were permitted to select their preferred ambient temperature (Ta). After one hour, the animals were removed from the gradient, injected with mouse urine at doses of 0, 6, 12, or 18 ml/kg (i.p.) and were placed quickly back in the gradient. The 18 ml/kg dose elicited a decrease in the selected Ta along with a significant reduction in body temperature measured at 60 and 120 min post-injection. In a second experiment it was found that urine collected from animals maintained at Ta's of 20, 30, or 35C overnight had similar effects on the behavioral thermoregulatory response. In the third experiment, mouse urine was dialyzed against saline using dialysis tubing with a molecular cutoff of 1000 daltons. The dialyzed urine had no effect on behavioral thermoregulation but did elicit a significant drop in body temperature; however, the decrease in temperature was not as great as that following administration of undialyzed urine. The studies indicate that mouse urine contains toxic substances that evoke a decrease in the set-point. Interestingly, the response to urine is similar to that following acute exposure to xenobiotic chemical substances.