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 decreased 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.