The subfornical organ is an essential central nucleus for angiotensin II-induced body fluid regulation. Similar to angiotensin II, centrally injected neurokinin B (NKB) may induce cardiovascular responses by the subfornical organ; however, it does not induce water intake. To clarify this inconsistency, we investigated the neuronal effects of NKB on subfornical organ neurons in slice preparations along with its behavioral effects in vivo. In electrophysiological extracellular recordings, NKB excited angiotensin II-insensitive and inhibited angiotensin II-sensitive neurons. Centrally injected NKB inhibited peripherally injected angiotensin II-induced water intake. These results suggest that NKB-mediated neuronal effects on the subfornical organ are likely to be involved in antidipsogenic responses in addition to the previously reported cardiovascular responses.