Activation of 5-HT_1A receptors in the medullary raphe decreases sympathetic outflow to thermoregulatory mechanisms, including brown adipose tissue (BAT) thermogenesis and peripheral vasoconstriction when these mechanisms are previously activated with leptin, prostaglandins, or cooling. These same mechanisms are also inhibited during REM sleep. It is not known whether shivering is also modulated by medullary raphe neurons. We previously showed in the conscious piglet that activation of 5-HT_1A receptors with 8-OH-DPAT (DPAT) in the paragigantocellularis lateralis (PGCL), a medullary region lateral to the midline raphe that contains 5-HT neurons, decreases heart rate, body temperature and muscle activity during NREM sleep. We therefore hypothesized that activation of 5-HT_1A receptors in the PGCL would also attenuate shivering and peripheral vasoconstriction during cooling. During REM sleep in a cool environment, shivering, carbon dioxide production and body temperature decreased, and ear capillary blood flow and ear skin temperature increased. Shivering associated with rapid cooling was attenuated after dialysis of DPAT into the PGCL. In animals maintained in a continuous cool environment, dialysis of DPAT into the PGCL attenuated shivering and decreased body temperature, but there were no significant increases in ear capillary blood flow or ear skin temperature. We conclude that both naturally occurring REM sleep and exogenous activation of 5-HT_1A receptors in the PGCL are associated with a suspension of shivering during cooling. Our data are consistent with the hypothesis that 5-HT neurons in the PGCL facilitate oscillating spinal motor circuits involved in shivering but are less involved in modulating sympathetically mediated thermoregulatory mechanisms.