With oral furosemide administration and salt loading, urinary calcium was significantly increased in 8 normal subjects, accompanied by parallel natriuresis. In spite of the excessive calcium loss in the urine, total and ionized serum calcium remained unchanged. All subjects had significant increases in nephrogenous cyclic AMP, suggesting that parathyroid activity is elevated in subjects with furosemide-induced hypercalciuria. With furosemide, fecal calcium was significantly decreased, and resultantly, there was no significant change in the cumulative calcium balance. It is suggested that urinary calcium loss with furosemide is compensated for by secondary hyperparathyroidism via increased intestinal calcium absorption in order to maintain serum calcium at a normal level. The experimental model thus mimics the condition of the renal type of idiopathic hypercalciuria.