1. Elevated concentrations of immunoreactive parathyroid hormone (P.T.H.) were detected in the pre-treatment plasma samples of seven out of ten children with nutritional rickets. The three normal values occurred in an inactive case, an infant with associated kwashiorkor and a child with late onset juvenile rickets. 2. Five children, reinvestigated after 6 weeks of vitamin D treatment, showed highly significant falls in P.T.H. concentration, confirming the functional nature of the secondary hyperparathyroidism. The activity of plasma alkaline phosphatase was less consistently decreased after treatment in these cases. 3. Convincing radiological evidence of (secondary) hyperparathyroidism was not detected in the hand bones of any of the rachitic children. 4. The possible diagnostic value of plasma P.T.H. measurement in active rickets is indicated.