Responsiveness of urinary cyclic AMP and phosphate to 200 units of parathyroid extract was evaluated in 5 normal subjects, 2 patients with idiopathic hypoparathyroidism, 4 patients with pseudohypoparathyroidism and 3 patients with primary hyperparathyroidism. Among them, 3 patients with pseudohypoparathyroidism were examined prior to and during therapy with vitamin D. Two patients with primary hyperparathyroidism were examined before and after removal of adenomas. In control subjects, percent increase in cyclic AMP after parathyroid extract administration was 7265 plus or minus 3312%, and in phosphate 290 plus or minus 72%. It was found that in idiopathic hypoparathyroidism the response of cyclic AMP was in the normal range, though that of phosphate was higher than normal. In pseudohypoparathyroidism, as distinguished from what Drezner et al. called pseudohypoparathyroidism type II, the response of cyclic AMP was uniformly low, while that of phosphate was variable. Similar results were obtained during treatment with vitamin D. In primary hyperparathyroidism, the responses of both cyclic AMP and phosphate were lower than normal. After removal of adenomas, the response of phosphate became norma., but the response of cyclic AMP rose to a subnormal level in one patient, and remained low in the other. For the diagnosis of pseudohypoparathyroidism, the response in cyclic AMP was considered to be a more reliable index than that in phosphate whether the patient was being treated with vitamin D or not.