An outbreak of muscle stiffness and poor performance among 59 thoroughbreds at a Newmarket flat racing yard was investigated between the beginning of May and the end of June 1986. Over a third of the horses showed signs of muscular stiffness, and 38 had, at one or more of the sampling times, creatine kinase (CK) activities above 200 iu/litre and, or, aspartate aminotransferase (AST) activities above 300 iu/litre when they were sampled six to eight hours after exercise. The following season, at a similar time and stage of training, only four of 39 horses sampled had CK activities between 200 and 300 iu/litre, and three had AST activities between 500 and 600 iu/litre. Plasma samples from 18 animals sampled at the start of the investigation and 13 days later were tested for the presence of antibodies to equine herpesvirus (EHV). Ten of them showed a response highly suggestive of an EHV-1 infection. No significant abnormality was found in the fractional electrolyte values from seven randomly selected animals. Unusually high numbers of normally sized and stained fibres with centrally placed nuclei as well as groups of small muscle fibres were found in muscle biopsies taken from three animals which continued to have high enzyme activities and show recurrent signs of muscle stiffness.