In a four-part study, we expand on our previous report that bulbospinal serotonin (5HT) neuronal activation occurs with 24 h of cold exposure. To characterize temporal aspects, rats were exposed to 3 degrees C or were maintained at 22 degrees C for 2, 8, 48, or 96 h (experiment 1) or for 15, 30, or 60 min (experiment 2). To ensure that cold-induced changes in 5HT activity were not due to disturbances in diurnal pattern, rats in experiment 3 were exposed to cold (8 h) during the dark cycle. To explore the hypothesis that cold-induced 5HT activation is part of a broad metabolic response that includes activation of the sympathetic nervous system, metabolically impaired (hypothyroid) rats were exposed to 8 degrees C in experiment 4. Significant increments in 5-hydroxyindoleacetic acid (SHIAA) concentration were evident by 60 min of cold exposure and existed at all later time points measured. These findings were most robust in spinal cord and rostral brainstem. Activation in spinal cord was also found when rats were exposed to 8 h of cold during the dark cycle, the active period for rats. In experiment 4, hypothyroid rats exhibited significantly greater norepinephrine excretion compared with control rats exposed to the same cold stimulus; this finding was accompanied by significantly greater increments in 5HIAA concentration in rostral brainstem and spinal cord of hypothyroid rats. In addition, significant elevations in tryptophan concentration were noted throughout the brainstem and spinal cord of cold-exposed, hypothyroid rats relative to room temperature, hypothyroid rats. This finding suggested that elevations in 5HIAA concentration in these rats were due to increases in precursor availability. The implications of these findings relative to autonomic and metabolic control are discussed.