Objectives - To determine maintenance energy requirements and effect of diet on performance of racing Greyhounds. Animals - 7 adult racing Greyhounds. Procedure - Dogs were fed a higher fat and protein (HFP) or a lower fat and protein (LFP) diet for 8 weeks in a crossover design. Dogs were exercised for 15 minutes twice daily in a paddock and raced 500 m twice weekly. Blood gas, hematologic, and serum biochemical analyses were performed before and after racing, and race times were compared at the end of each diet period. Results - Mean race time was significantly shorter (32.81 ± 0.65 seconds vs 33.05 ± 0.71 seconds), and mean racing speed over 500 m was significantly faster (15.25 ± 0.30 vs 15.13 ± 0.30 m·s-1) when dogs were fed the HFP diet than when they were fed the LFP diet. Diet had little or no effect on results of blood gas, hematologic, and serum biochemical analyses, except that Hct was 4% greater before and after racing when the HFP diet was fed than when the LFP diet was fed. Mean SD metabolizable energy intake from weeks 1 through 16 was 155 ± 9 kcal·kg-0.75·d-1. Conclusions and Clinical Relevance - Racing Greyhounds ran faster when fed a diet containing higher fat and protein and lower carbohydrate contents. Their maintenance metabolizable energy requirement was slightly higher than that of moderately active dogs.