Brown adipose tissue (BAT) is thought to be responsible for increased heat production in cold-acclimated rodents. We measured sympathetic nerve activity (SNA) in interscapular BAT (IBAT) during cold stimulation in cold-acclimated C57BL/6J mice (ACCLI). Cold acclimation was achieved (cold tolerance was increased) by repeated exposure to cold stress every other week for 3 weeks. We compared SNA in these animals with SNA in mice that had no previous cold stress experience (naive). During the test, mice were anesthetized by urethane and isoflurane and were paralyzed with vecuronium bromide. Sympathetic nerve activity was recorded directly from one of the fine nerves to IBAT. The animal's body caudal to the pelvic area was covered with a plastic bag containing a slurry of ice water to decrease colonic temperature 7 degrees C below control level, which took approximately 20 min. Interscapular BAT-SNA increased during cold stress in both groups, but ACCLI mice had higher IBAT-SNA during cold stress than naive mice. These findings confirmed the hypothesis that during the acute cold exposure, cold-acclimated mice have greater sympathetic outflow to BAT adipocytes.