SUMMARY: The effect of salt-loading, thirst and water-loading on the axonal transport and turnover of neurohypophysial proteins in rats was investigated by measuring the radioactivity of the neural lobe proteins at various time intervals after injection of [35S]cysteine into the region of the supraoptic nucleus. Osmotic stimuli did not markedly affect the rate of transport of either the rapid or the slow phase of axonal transport of proteins. In dehydrated rats the increase of neurohypophysial radioactivity during short time intervals after isotope injection exceeded that in controls, demonstrating an increased axonal transport due to osmotic stimuli. The turnover of rapidly as well as slowly transported neurohypophysial proteins was increased after dehydration but was decreased in water-loaded rats. After water deprivation or salt-loading with 2% NaCl solution for 7 days the neural lobe neurophysin was reduced in amount by 80–90%, whereas during salt-loading with 1% NaCl solution a 20% reduction was observed on the 14th as well as the 30th day. As judged from the amount of radioactivity in posterior pituitary neurophysin in dehydrated rats it is suggested that newly synthesized rapidly transported neurophysin is rapidly released after its arrival in the neural lobe.