In response to stress, cells synthesise heat shock proteins (HSP) to maintain protein homeostasis. To study whether exercise and training induce expression of HSP72 in the middle gluteal muscle, 10 Finnhorses performed a submaximal 60 min exercise test on a treadmill. Test A was performed after 3 months of training, and the other two tests 2 (B) and 5 (C) weeks later. Blood samples were taken during and after the tests, and biopsy samples before, immediately after and 23 h after each test. HSP72 mRNA was analysed using a digoxigenin-labelled probe. Blood lactate concentration in the 3 tests varied between 7.2 and 10.2 mmol/I. Training increased HSP72 mRNA, as indicated by increases in samples taken at rest (A<B<C). Exercise also tended to increase HSP72 mRNA transiently but, 23 h later, values had returned to pre-exercise levels. HSP72 mRNA was expressed in all muscle fibres. After exercise, HSP72 mRNA correlated positively with the peak concentration of blood lactate, but not with indicators of energy status. Therefore, acidosis rather than energy depletion was the major inducer of HSP72 expression after moderate intensity exercise. Because HSP72 may protect cells against stress, knowledge about their expression may help in planning optimal trainng regimes.