We have investigated the effects of a reduced regional or a reduced body temperature, on pial arteriolar response to a variety of vasoactive stimuli. This has been done by studying the effects of these stimuli on groups of mice with different body temperatures, or with different temperatures of the artificial cerebrospinal fluid (CSF) irrigating the pial surface. Responses in mice with body temperatures of 30° or 22°C showed little or no difference from responses in mice with body temperatures of 37°. This was so whether the surface irrigant was maintained at 37° or at 23°. On the other hand, significant reductions in pial vascular responses were observed when mice with "CSF" temperatures of 22° were compared with those having "CSF" temperatures of 37°. The data suggest that regional cooling is more effective than cooling the body in reducing the responses of pial arterioles. The data also indicate that marked reductions in body temperature would have to occur before a detectable effect on pial vascular responses is produced, at least in mice anesthetized with urethane.