To date, all known in vivo effects of icilin are dependent on TRPM8 , yet genetic evidence demonstrating that icilin-induced hyperthermia is TRPM8-dependent is yet to be established. Therefore, we first examined the role of TRPM8 activation in thermoregulatory SB203580 biological activity responses by subcutaneously injecting 10 mg/kg icilin into order Tubacin wildtype and TRPM8-knockout mice implanted with thermal telemeters . Consistent with data in rats , we observed a pronounced hyperthermic effect of 1.6uC on average in wildtype mice, which resolved within 90 minutes . However, this hyperthermic response was absent in TRPM8-/- mice, with only a small injection-related artifact observed that was similar to vehicle injections . When we administered 1 mg/kg capsaicin s.c. to wildtype and TRPM8-/- mice we found a profound and transient hypothermic effect of around 4uC that was similar in both genotypes, indicating that the TRPM8-/- mice were still able to mount a chemically-induced thermoregulatory response . Injection of the DMSO/saline vehicle s.c. induced only a brief increase in body temperature of around 0.5uC which peaked within 30 minutes post-injection in both genotypes . We next determined if PBMC antagonism of TRPM8 alters thermoregulatory responses in a likewise, yet reversed, manner. However, we found that subcutaneous injections of the required vehicle for PBMC ) resulted in intense grooming and scratching at the site of injection in both wildtype and TRPM8-/- mice. Since stress is known to influence thermoregulation , we therefore switched to intraperitoneal injections of solutions warmed to 37uC immediately before injection and administered as far away from the telemeter implantation site as possible. This approach resulted in no obvious adverse effects associated with intraperitoneal vehicle injections . Next, we tested a range of PBMC doses , finding no effect with 2 mg/kg and a small, but significant drop in core body temperature with 10 mg/kg which peaked at 0.8uC below baseline by two hours post-injection . Strikingly, at 20 mg/kg, we observed a dramatic and severe hypothermic effect of more than 6uC, with a drop in core body temperature to below 30uC in one instance . The drop in core body temperature of more than two degrees lasted at least four hours on average. Importantly, TRPM8-/- mice showed no fluctuations in core temperature besides the transient injection artifact at all doses . These data show that blockade of TRPM8 activity at high PBMC doses significantly alters thermoregulation, providing pharmacological evidence that, like TRPV1 , TRPM8 is involved in the maintenance of core body temperature. We and others have previously reported that TRPM8 is required for behavioral responses to cooling over a broad range of cold temperatures . To test whether pharmacological blockade of TRPM8 channels by PBMC affects normal thermosensation, we gave mice intraperitoneal injections of 10 or 20 mg/ kg PBMC and assayed cold thermosensation at one hour postinjection using the evaporative cooling assay .