A specific and sensitive radioimmunoassay for human neurophysin has been developed, using H-NpI as the human neurophysin standard, anti-H-NpI as the antiserum and HNpI-131I as the tracer. The serum concentration of neurophysin under basal conditions in patients of different sex and age was 2.0 ± 1.0 mμg/ml. During pregnancy, serum neurophysin increased progressively, reaching a mean of 7.6 ± 2.6 mμg/ml. Amniotic fluid neurophysin levels at term were 11.1 ± 5.6 mμg/ml. In the postpartum period, the concentration of serum neurophysin decreased towards basal values but no significant change was noted before, during and after nursing. In newborns, serum neurophysin was 9.2 ± 6.0 mμg/ml. In some patients with galactorrhea, acromegaly and inappropriate secretion of ADH, the serum concentration of neurophysin was elevated. Serum neurophysin values in patients with diabetes insipidus were generally elevated. Patients with renal failure, had a mean serum neurophysin concentration of 6.0 ± 2.4 mμg/ml. The molecular size, immunological cross-reactivity and electrophoretic mobility of neurophysin derived from the serum of normal subjects and of patients with pituitary or hypothalamic disorders was similar. However, the relative amounts of the two components of human neurophysin, H-Npl and H-NpII, varied slightly in serum samples obtained from patients with different pathological conditions.