Despite the widespread chronic use of furosemide in the clinical management of a variety of fluid and electrolyte disorders in human infants, the physiological responses to furosemide in the newborn after chronic furosemide treatment are not known. The present experiments were conducted to determine the effects of chronic furosemide treatment on renal responses to acute furosemide challenge in conscious, chronically instrumented lambs. Experiments were carried out on day 1 (before chronic treatment) and on day 7 (after chronic treatment) in lambs given intravenous injections of either furosemide (1 mg/kg per 12 h for 5 days, n=9) or vehicle (0 mg/kg per 12 h for 5 days, n=4). Furosemide-treated animals responded to acute furosemide challenge on day 7 with attenuation of natriuresis and diuresis, and augmentation of kaliuresis compared with responses on day 1. Baseline renin production was elevated, although the renin response to furosemide was similar in chronic furosemide-treated and vehicle-treated lambs. Baseline aldosterone levels were not altered by chronic furosemide treatment, but the aldosterone response to acute furosemide injection was decreased after chronic furosemide treatment. Therefore, chronic furosemide treatment alters renal responses to furosemide in conscious lambs, and alters the aldosterone response to acute furosemide challenge.