Two experiments were conducted to investigate the effect of conditioning on the ability of the equine to digest and utilize nutrients and to determine the effect of dietary fat as an energy source on the physiological parameters associated with fitness using a standard exercise test. Conditioning horses increased apparent digestibilities of crude protein (CP) (P<.05), dry matter(DM), and acid detergent fiber (ADF) (P<.1). Conditioning also tended to increase the apparent digestibility of neutral detergent fiber (NDF), cellulose, cell contents, and energy. Heart rates and blood lactate levels indicated that the conditioned horses were fitter than their unconditioned controls. In the second experiment, horses were fed isocaloric diets, one containing added fat and the other a standard hay/corn diet. Adding fat while maintaining equal available energy concentration depressed apparent digestibility of dry matter (56.7 vs 67.3 % P<.05), cell contents (75.6 vs 82 %, P<.05), energy (61.2 vs 71.8 %, P<.05) and NDF (29.2 vs 51.3 %, P<.05), in unconditioned horses. There was a trend towards decreased apparent digestibility of CP and ADF. Addition of fat increased apparent digestibility of ether extract (89.2 vs 65.6 %, P<.05). Conditioning increased apparent digestibility of CP (64.8 vs 73.7 %, P<.05) and energy (61.2 vs 65.6 %, P<.05) and tended to increase apparent digestibility of DM (56.7 vs 60.8 % ) and ADF (26.8 vs 17.8 %) for horses fed a fat supplemented diet. Conditioning did not cause a change in apparent digestibility of ADF, CP, and DM in horses fed the control diet, or apparent digestibilities of NDF, ether extract, cell contents, or energy for either diet. There were no differences in physiological parameters used for assessing fitness (heart rate, blood lactate, and respiration rate), between horses fed a diet containing 14% added fat and no added fat. There was no difference in body temperature, blood glucose levels, blood urea-N (BUN), or creatine phosphokinase (CPK) between horses fed the two diets. / Ph. D.