Blood samples were collected twice weekly over a nine month period from 24 Thoroughbred racehorses in training at Newmarket to study the effects of daily training schedules and stage of oestrous cycle on serum enzyme levels and clinical signs of equine exertional myopathy. The sampling period extended from November to July and was performed between 16:00 h and 18:00 h so as to be at least 6 h after exercise. Serum creatine kinase, aspartate aminotransferase and plasma progestogen concentrations were measured. All horses were subjected to a similar traditional training programme. The study demonstrated marked fluctuations in serum muscle enzyme concentrations which were more pronounced in two-year-old fillies than three-year-old fillies and colts. There was no correlation, however, with the stage of oestrous cycle. It was concluded that some degree of muscle cell damage, which results in a variable increase in serum creatine kinase levels, occurs in all horses following light exercise after a rest day. Traditional flat race training may not be ideal in this respect and it is suggested that, for horses with an observed tendency towards exertional myopathy, more consistent and longer periods of less strenuous exercise may be advantageous.