The central mechanism of the pressor response induced by stimulation of the central ends of abdominal vagi (SCAV) was studied in 33 anesthetized cats. The results of the experiments showed that both electro-stimulation to part of the nucleus tractus solitarius (NTS) and SCAV caused similar rises of blood pressure. The microinjection of kainic acid into the NTS (40 mmol/L, 2 microliters) evoked a prolonged rise of blood pressure but did not affect the pressor response induced by SCAV. Electro-stimulation of the locus coeruleus (LC) also elicited a pressor effect. Kainic acid at blocking dose (40 mmol/L, 2 microliters bilaterally injected into the LC induced an increase followed by a decrease in blood pressure and markedly attenuated the SCAV's pressor response. Bilateral microinjections of phentolamine (4 micrograms/2 microliters), propranolol (2 micrograms/2 microliters) or 6-hydroxydopamine (6-OHDA, 8 micrograms/2 microliters) into LC had no significant effect on the pressor response, but atropine could markedly attenuate it. These results suggest that the pressor afferents in abdominal vagi do not terminate in but pass through the NTS and the neurons in the LC play an important role in the SCAV pressor reflex. Cholinergic transmission rather than catecholaminergic transmission in the LC may be involved in this pressor response.