Abstract Vasopressin ribonucleic acid (VP RNA) accumulates in the posterior lobe of the rat pituitary in response to an osmotic stimulus. The accumulation of posterior pituitary VP RNA can be prevented by stalk transection or the intracerebroventricular injection of colchicine. The hypothalamic and pituitary VP RNAs however, have different structures and are independently regulated. Depletion of serotonin with parachlorophenylalanine blocks the osmotically induced accumulation of both VP and oxytocin RNA in the hypothalamus but does not affect the accumulation of VP RNA in the posterior pituitary gland. In Brattleboro rats, posterior lobe oxytocin RNA but not VP RNA, increases after osmotic stimulation. Although axonal transport of RNA may occur, our results are also consistent with VP RNA synthesis in pituicytes controlled by hypothalamic neuroendocrine factors released by neurohypophyseal terminals in response to osmotic stimuli.