Five patients with pseudohypoparathyroidism were compared to normal subjects and patients with hypoparathyroidism in their ability to respond to the infusion of parathyroid hormone (PTH) by altering excretion of calcium, sodium, potassium, phosphate and bicarbonate. In patients with pseudohypoparathyroidism, impairment in renal responses to PTH was more generalized than has been recognized. The patterns of response varied from patient to patient. The most commonly observed abnormality, aside from lack of increase in urinary cyclic adenosine 5'-monophosphate (AMP) was failure to decrease the calcium to sodium clearance ratio, and indication of impaired renal calcium reabsorption. The responses which most closely approximated normal, including a normal decrease in the calcium to sodium clearance ratio, occurred in a patient (Case 1) who had the largest, although impaired, response in cyclic AMP excretion. Conversely, the most abnormal responses occurred in three patients (Cases 2, 4 and 5) who had the smallest increases in cyclic AMP excretion after the administration of PTH. The impaired renal reabsorption of calcium after the administration of PTH (lack of decrease in calcium to sodium clearance ratio) may, when present, be in part responsible for hypocalcemia.