Hinchcliff, K. W., K. H. McKeever, W. W. Muir, and R. A. Sams. Furosemide reduces accumulated oxygen deficit in horses during brief intense exertion. J. Appl. Physiol. 81(4): 1550-1554, 1996.We theorized that furosemide-induced weight reduction would reduce the contribution of anaerobic metabolism to energy expenditure of horses during intense exertion. The effects of furosemide on accumulated O₂ deficit and plasma lactate concentration of horses during high-intensity exercise were examined in a three-way balance randomized crossover study. Nine horses completed each of three trials: 1) a control (C) trial, 2) a furosemide-unloaded (FU) trial in which the horse received furosemide 4 h before running, and 3) a furosemide weight-loaded (FL) trial during which the horse received furosemide and carried weight equal to the weight lost after furosemide administration. Horses ran for 2 min at ~120% maximal O₂ consumption. Furosemide (FU) increased O₂ consumption (ml ^· 2 min^{1 ·} kg¹) compared with C (268 ± 9 and 257 ± 9, P < 0.05), whereas FL was not different from C (252 ± 8). Accumulated O₂ deficit (ml O₂ equivalents/kg) was significantly (P < 0.05) lower during FU (81.2 ± 12.5), but not during FL (96.9 ± 12.4), than during C (91.4 ± 11.5). Rate of increase in blood lactate concentration (mmol ^· 2 min^{1 ·} kg¹) after FU (0.058 ± 0.001), but not after FL (0.061 ± 0.001), was significantly (P < 0.05) lower than after C (0.061 ± 0.001). Furosemide decreased the accumulated O₂ deficit and rate of increase in blood lactate concentration of horses during brief high-intensity exertion. The reduction in accumulated O₂ deficit in FU-treated horses was attributable to an increase in the mass-specific rate of O₂ consumption during the high-intensity exercise test.