1. Rat neurohypophysial extracts have been examined by polyacrylamide-gel electrophoresis. 2. Three of the proteins were tentatively identified as neurophysins by their acidic nature and their disappearance after dehydration of the animals. 3. These proteins were radioactive 24h after intracisternal injection of [(35)S]cysteine. 4. Two of the proteins were present in much greater quantities than the third, and these two were present in the gland in the same ratio as the hormones vasopressin and oxytocin. 5. One of these proteins was absent from glands of rats homozygous for diabetes insipidus but present in heterozygous animals. 6. It is suggested that these two proteins are the vasopressin-neurophysin and oxytocin-neurophysin of the rat.

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1. Radioactivity associated with the three neurophysins in the neural lobe of the rat was determined at intervals up to 5 weeks after an intracisternal injection of [(35)S]cysteine. 2. The radioactivity associated with the two major neurophysins (one supposedly associated with vasopressin and the other with oxytocin) increased linearly for 12h after the injection and the ratio of the rates of increase in the two proteins was very similar to the ratio of vasopressin to oxytocin in the gland. 3. From 12h onwards the radioactivity associated with each major neurophysin declined exponentially but the half-life of the supposed oxytocin-neurophysin (13.3 days) was shorter than that for the supposed vasopressin-neurophysin (19.8 days). 4. The kinetics of labelling of the minor neurophysin was quite different from that of the two major ones. It became slowly labelled during 3-5 days after injection and the radioactivity hardly decreased during the following 4 weeks. 5. The data could support the hypothesis that the minor neurophysin is a metabolic product of oxytocin-neurophysin. The exponential rate of disappearance of radioactivity from oxytocin-neurophysin and the minor component taken together has a rate constant similar to that for vasopressin-neurophysin (e.g. half-life=18.9 days).

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The content of vasopressin, oxytocin and their related neurophysins was measured in the hypothalamus and pituitary gland of mid-trimester human fetuses. Vasopressin was present in both tissues approximately 3-4 weeks before oxytocin. The levels of the hormones in the pituitary gland increased 1000-fold over the next 3-4 months. During this time, the very high vasopressin/oxytocin ratio gradually deceased but did not reach unity in the period studied. In contrast, both the vasopressin-associated neurophysin and the oxytocin-associated neurophysin appeared in the pituitary gland at the same gestational age and showed the same exponential increase with fetal age. Lower levels of the neurophysins and the nonapeptides were found in the hypothalamus and the levels increased more slowly with fetal age. Our results suggest that the high vasopressin/oxytocin ratios observed in fetal life are due to differences in the rate of maturation of the hormone precursor, rather than to differences in the rate of de-novo synthesis.