The PRL response to the infusion of 8 U/kg BW parathyroid extract (PTE) was studied in three children with pseudohypoparathyroidism type I (PHP), three patients with idiopathic hypoparathyroidism, and eight epileptic children. PTE produced a clear-but increase of serum PRL in the epileptics and patients with idiopathic hypoparathyroidism, but not in the children with PHP. Prolonged PTE administration (six doses of 4 U/kg BW over 48 h) was also without effect on the low serum PRL levels in the three patients with PHP. These patients exhibited an isolated diminished PRL reserve to other potent PRL stimuli in the presence of intact function of the remainder of the anterior pituitary. The data suggest a pituitary receptor defect in patients with PHP in addition to disturbed renal and bone responses. One patient with normocalcemic PHP, seeming to be in the early stage of the disease, demonstrated the same distinct PRL deficiency as the other two patients with manifest PHP. This suggests that the pituitary receptor defect may be an early sign and marker of PHP.