To assess whether GABA given intracerebroventricularly (i.c.v.) affects vasopressin (AVP) and atrial natriuretic peptide (ANP) release and changes in blood pressure in response to i.c.v. angiotensin (AT II) and carbachol (CB), or whether changes in blood pressure affect GABA release in the brain, experiments were carried out. In experiment I (Ex I), GABA (100 μg) with AT II (50 ng) or CB (25 ng) was i.c.v. administered in conscious rats ( n = 12). The same dose of AT II or CB alone also was administered without GABA ( n = 12). In experiment II (Ex II), AT II (100 ng/kg per min) or nitropuruside (NP, 10 μg/kg per min) was intravenously (i.v.) infused and GABA release in the area adjacent to the paraventricular nucleus was determined, using the microdialysis method, in conscious rats ( n = 12). In the experiments, mean artrial blood pressure (MABP), heart rate (HR), plasma AVP and/or ANP and plasma Na + and K + levels were measured. In Ex I, i.c.v. AT II increased plasma AVP and MABP without changes in HR and plasma ANP, but i.c.v. GABA never affected these responses. Icv CB also increased plasma AVP and MABP with decreased HR, but did not affect plasma ANP. Icv GABA abolished bradycardiac responses, but did not affect the others. In Ex II, the pressor response to i.v. AT II increased GABA release without apparent decreases in plasma AVP. However, the depressor response to NP produced decreases in GABA release with increased plasma AVP. These results shows that i.c.v. GABA did not affect AVP and pressor responses to i.c.v. AT II and CB, but changes in blood pressure modulates GABA release in the brain with changes in plasma AVP.