Intravenous administration of the selective 5-HT2 agonist, 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), produced an increase in sympathetic activity recorded from the inferior cardiac nerve in chloralose-anesthetized cats. Microiontophoretically applied 5-HT increased the firing rate of antidromically identified sympathetic preganglionic neurons. Microiontophoretic DOI failed to affect the firing of sympathetic preganglionic neurons. The effect of DOI was also studied on medullospinal sympathoexcitatory neurons located in the rostral ventrolateral medulla. Intravenous DOI increased the firing of sympathoexcitatory neurons and sympathetic nerve discharge to a similar extent. Microiontophoretic application of DOI failed to affect the firing of sympathoexcitatory neurons. The data are discussed in relation to the site and mechanism of the sympathoexcitatory action of 5-HT2 agonists.