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Experiments were designed to determine in the same animal whether electrical stimulation of the posterior hypothalamus and ventromedial hypothalamic nucleus could specifically evoke shivering and nonshivering (brown adipose tissue) thermogenesis, respectively, in anesthetized, normothermic rats. Urethane-anesthetized, male Long-Evans rats, kept at 37 degrees C, had colonic (Tc), gastrocnemius muscle (Tm), intrascapular brown adipose tissue (TIBAT), and tail (Tt) temperatures measured via thermistor probes, and electromyogram activity (differential multiunit activity from bipolar recording electrodes within gastrocnemius muscle) recorded, before and after unilateral electrical stimulation (monophasic 0.5-ms pulses of 200 microA at 50 Hz for 30 s) of the posterior hypothalamus and ventromedial hypothalamic nucleus (via stereotaxically implanted concentric stimulating electrodes). Each rat showed shivering (increased electromyogram activity) following posterior hypothalamic stimulation, which caused an immediate rise in Tm values with no change in TIBAT or Tt values. Electrical stimulation of the ventromedial hypothalamic nucleus of the same animals elicited no shivering activity, but significant increases in TIBAT values occurred with no change in Tm or Tt values. Results confirm that stimulation of the posterior and ventromedial hypothalamic nuclei in rodents specifically activates shivering and nonshivering (brown adipose tissue) effector mechanisms, respectively, to raise core temperature.