Serotonergic single-unit activity during glucoregulatory challenges was studied in the nuclei raphe obscurus (NRO) and raphe pallidus (NRP) of freely moving cats. Systemic insulin administration (2-4 IU/kg, i.v.) suppressed neuronal activity by approximately 40% in direct relationship to blood glucose levels and in inverse relationship to plasma catecholamine levels. NRO and NRP serotonergic neurons displayed a temporary recovery in unit activity in response to i.v. glucose administration (500 mg/kg), which temporarily reversed insulin-induced hypoglycemia. The neuronal responses to insulin and subsequent glucose administration were also directly related to changes in integrated nuchal electromyographic activity. Serotonergic unit activity remained unchanged after glucose loading (500 mg/kg, i.v.), which produced a four-fold increase in blood glucose. Thus, medullary serotonergic neurons appear to be sensitive to reductions, but not increases, in blood glucose. The observed inverse relationship between unit activity and plasma catecholamines does not support a postulated sympathoexcitatory role for these neurons. Instead, the parallel changes in single-unit activity and integrated muscle activity support the hypothesis that the activity of medullary serotonergic neurons is linked to motor output. These neurons may modulate autonomic outflow, but only in relationship to their primary role in motor control. Finally, medullary serotonergic neurons may play a protective role in maintaining glucose homeostasis by disfacilitating the output of the somatomotor system, and hence diminishing muscle energy demands, when peripheral glucose availability is low.