Body-fluid homeostasis is essential to life, and the concentration of Na⁺ ([Na⁺]) and osmolality in plasma and the cerebrospinal fluid (CSF) are continuously monitored in the brain. To maintain a physiological level of Na/osmolality in body fluids, the control of Na and water intake and excretion are of prime importance. Two independent sensing systems for [Na⁺] and osmolality in circumventricular organs (CVOs) have long been postulated to be involved in the monitoring of body-fluid conditions. In the past decade, several molecules were reported as promising candidates for these sensors – Na_x for the [Na⁺] sensor and transient receptor potential (TRP) channels for the osmosensor. This review presents a summary of developments in these areas over recent years.