Extracellular recordings were obtained from 32 phasically active neurosecretory cells in the hypothalamic paraventricular nucleus (PVN) of urethane-anesthetized male rats. None of the PVN cells changed their activity to intracarotid infusions of isotonic saline (0.15 M NaCl solution, 0.05 ml). Of these PVN neurons, 26 displayed an increase in neuronal activity following intracarotid infusions of hypertonic saline (0.2 M NaCl solution, 0.05 ml), while the remainder were unresponsive. Microinjection of the local anesthetic lidocaine into the subfornical organ (SFO) reversibly diminished the excitatory response to the infusions of hypertonic saline in 10 out of 15 PVN neurons tested, whereas the injection of lidocaine into the vicinity of the SFO (n = 4) or the third ventricle (n = 4) did not cause a marked change. These results show an involvement of the SFO in the mechanism of osmotic activation of putative vasopressin (AVP)-secreting neurons in the PVN.