The effects of various osmotic agents on the release of oxytocin were examined in a superfusion system. Oxytocin was released significantly from the rat pituitary by superfusion with medium of an osmolality of 350mOsm/kg H2O adjusted with NaCl, regardless of the presence of the rat hypothalamus. Media adjusted to an osmolality of 350 mOsm/kg H2O with sucrose, glucose, urea or mannitol had no effect on oxytocin release from the hypothalamo-pituitary complex. Medium containing excess Na2SO4 induced significant release of oxytocin from the pituitary without the hypothalamus. The administration of tetraethlyammonium chloride had no oxytocin secretion. These data suggest that oxytocin release from the pituitary is influenced by the level of sodium ion rather than the osmotic pressure.