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SUMMARY A controlled period of submaximal exercise on a treadmill was used as a standardised stress test in 6 young horses to monitor the effects of training. Circulating plasma concentrations of immunoreactive beta-endorphin (IR β-EP) were measured before, during and after the exercise period. The stress test was conducted on 3 occasions during an intensive training program lasting 14 weeks. In week 3 a marked increase in plasma IR β-EP (P=0.003) was demonstrated as a result of training, but by the last exercise test performed in week 9 no significant increase in plasma IR β-EP concentrations could be detected. During the training period the basal concentrations of plasma IR β-EP significantly decreased (P=0.0059). Plasma adrenocorticotrophin (ACTH) did not increase during exercise, although there was a trend of decreasing basal plasma ACTH by the end of the training period.

It was concluded that a standardised work test acted as a mild stress to unfit horses, but as the horses' fitness increased the hormonal response to exercise diminished. Basal plasma β-EP concentrations were decreased with increasing fitness.