Background: Horse racing is a unique weight category sport whereby the handicapping system allocates a specific body mass which the jockey must attain for each race. As a consequence rapidly reducing body mass prior to competition appears to be an integral part of horse racing. This leads to jockeys practising potentially unsafe methods of rapid weight loss. Purpose: The purpose of this study was to investigate the effect of rapid weight loss on jockey’s cognitive function and subjective well-being on a competitive race day. Methods: Twelve professional jockeys (see Table 1 for characteristics) completed a series of cognitive function tests (CogState Sport) and ratings of subjective well-being (Visual Analogue Scale) on two separate occasions, riding at either a comfortable body mass (61.8 ± 5.6kg) or having practiced rapid body mass loss strategies (58.3 ± 5.3kg) (-5.72 ± 1.9% body mass). Data are reported as Mean ± S. Results: The magnitude of rapid weight loss observed had no effect on cognitive function (see Figure 1), however, subjective well-being was impaired for feelings of hunger (p=0.05), mouth dryness (p=0.08), mouth taste (p=0.038), concentration (p=0.013) and alertness (p=0.04) (see Figure 2). Discussion: These results indicate that a ~6% reduction in body mass using rapid weight loss methods impaired subjective well-being but had no effect on cognitive function. Conclusion: The practice of rapid body mass loss in restricted sports such as horse racing may be unsafe and detrimental to athlete’s well-being.