We studied the effects of oral furosemide, 80 mg/day for 7 days, on the response of urinary excretion of phosphate and cyclic AMP to exogenous parathyroid extract (PTE) in 6 normal subjects. All 6 subjects had marked increases in urinary calcium and a significant increase in urinary cyclic AMP from the control to the furosemide periods: this suggests that furosemide-induced hypercalciuria produced elevated parathyroid activity. After treatment with furosemide, the response of urinary cyclic AMP and phosphate to PTE was blunted. During the subsequent calcium infusion (4 mg/kg), urinary cyclic AMP was suppressed to subnormal values, and the response to PTE returned to normal. The evidence suggests that furosemide may blunt the response to PTE, perhaps as a result of the elevated parathyroid activity produced by furosemide-induced hypercalciuria and lowering of plasma-ionized calcium. This blunting effect of furosemide on the response of urinary phosphate and cyclic AMP to PTE should be considered in the evaluation of parathyroid function in patients taking furosemide.