Stimulation of the insular cortex elicits a number of autonomic responses. The insular cortex projects directly to the lateral hypothalamic area, the parabrachial nucleus, and the nucleus of the solitary tract, which in turn project directly to sympathetic preganglionic areas. To determine which of these subcortical sites mediates sympathetic responses evoked from the insular cortex, changes in renal nerve activity were recorded before and after injection of the synaptic blocking agent cobalt into each of these regions. Blood pressure, heart rate, and renal nerve activity were continuously monitored in chloralose or urethan-anesthetized rats. Single-pulse electrical stimulation (200 microA, 1 ms) elicited either an early increase or decrease in renal nerve activity from pressor and depressor sites, respectively, in the insular cortex. Cobalt injections (500 nl) into the lateral hypothalamic area attenuated the nerve response 10-100%. Cobalt injections into the nucleus of the solitary tract significantly enhanced the initial increase in the nerve response obtained from pressor sites in the insular cortex. Injections into the parabrachial nucleus did not affect the nerve responses. These results suggest that there is a mandatory synapse in the lateral hypothalamic area in the pathway from the insular cortex to the sympathetic nervous system.