Body fluid provides the optimum environment for cells to maintain their activities. As cells cannot stop their activities even for a moment, the environment must not be disturbed. Water absorption across the ileum has been reported to be reflexively regulated. As Na + is one of important determinants for the body fluid volume and also an element for the excitable cells, it can be supposed that Na + metabolism is also reflexively regulated. The kidney has been regarded to be the main effector organ responding in negative feedback fashion to changes in the body fluid and plasma Na + concentration. However, we raised two questions: (i) whether this negative feedback control system is ideal for the body fluid and Na + homeostasis, because these variables must have been changed prior to starting its operation and (ii) whether the kidney is the main effector organ in Na + homeostasis. If a control system senses a disturbance, predicts a change that will be caused by the disturbance and makes the effector organs cancel the change, this must be an ideal control system. This type of control system can be called a negative feedforward control system. Recently, we have found the hepato-renal and hepato-intestinal reflexes. They play an important role in body fluid and Na + homeostasis and are thought to be a negative feedforward control system. In Na + homeostasis, the main effector organ is the intestine, as well as the kidney. In this review, we describe two novel negative feedforward body fluid control systems and discuss their significance in Na + homeostasis.