Glutamate and aspartate excited all spontaneously active sympathetic preganglionic neurons (SPNs) tested in the intermediolateral nucleus of spinal segments T1-T3. Most silent neurons could be induced to discharge but the remainder showed only a decrease in antidromic spike amplitude. These effects were typically fast (on, off less than 1 s). D,L-Homocysteic acid also produced excitation; this effect was typically slower. Glutamate and aspartate were usually equipotent; 20% were differentially sensitive to aspartate, 10% to glutamate.