Activation of 5-HT 1A receptors in the medullary raphé decreases sympathetically mediated brown adipose tissue (BAT) thermogenesis and peripheral vasoconstriction when previously activated with leptin, LPS, prostaglandins, or cooling. It is not known whether shivering is also modulated by medullary raphé 5-HT 1A receptors. We previously showed in conscious piglets that activation of 5-HT 1A receptors with (Ϯ)-8-hydroxy-2-(dipropylamino)-tetralin (8-OH-DPAT) in the paragigantocellularis lateralis (PGCL), a medullary region lateral to the raphé that contains substantial numbers of 5-HT neurons, eliminates rapid eye movement (REM) sleep and decreases shivering in a cold environment, but does not attenuate peripheral vasoconstriction. Hoffman JM, Brown JW, Sirlin EA, Benoit AM, Gill WH, Harris MB, Darnall RA. Am J Physiol Regul Integr Comp Physiol 293: R518 -R527, 2007. We hypothesized that, during cooling, activation of 5-HT 1A receptors in the medullary raphé would also eliminate REM sleep and, in contrast to activation of 5-HT 1A receptors in the PGCL, would attenuate both shivering and peripheral vasoconstriction. In a continuously cool environment, dialysis of 8-OH-DPAT into the medullary raphé resulted in alternating brief periods of non-REM sleep and wakefulness and eliminated REM sleep, as observed when 8-OH-DPAT is dialyzed into the PGCL. Moreover, both shivering and peripheral vasoconstriction were significantly attenuated after 8-OH-DPAT dialysis into the medullary raphé. The effects of 8-OH-DPAT were prevented after dialysis of the selective 5-HT 1A receptor antagonist WAY-100635. We conclude that, during cooling, exogenous activation of 5-HT 1A receptors in the medullary raphé decreases both shivering and peripheral vasoconstriction. Our data are consistent with the hypothesis that neurons expressing 5-HT 1A receptors in the medullary raphé facilitate spinal motor circuits involved in shivering, as well as sympathetic stimulation of other thermoregulatory effector mechanisms. thermoregulation; serotonin; brain stem; raphé; the sudden infant death syndrome MANY NEURONS IN THE MEDULLARY raphé, including serotonergic (5-HT) neurons, project to the intermediolateral cell column (IML) of the spinal cord and modulate sympathetic outflow to thermoregulatory effector mechanisms, including brown adipose tissue (BAT) thermogenesis, vasoconstriction of thermoregulatory cutaneous vascular beds, and heart rate (HR) (14,53,58,86,89). Studies in anesthetized and conscious animals have shown that activation of 5-HT 1A receptors in the medullary raphé with the selective agonist, (Ϯ)-8-hydroxy-2-(dipropylamino)-tetralin (8-OH-DPAT), attenuates sympathetic outflow to BAT and peripheral vessels when previously elevated by LPS, leptin, or cooling (11,52,59,63,64).There is mounting evidence that medullary raphé 5-HT neurons modulate sympathetically mediated thermoregulatory mechanisms; however, little is known about the role of medullary raphé 5-HT neurons in shivering thermogenesis. Shivering is an involuntary tremor that i...