This experiment was undertaken to determine whether there were differences in cardiorespiratory, haematological and muscular responses in horses trained at either low or moderate intensities. Ten Thoroughbred horses previously rested in paddocks for 4 months were trained 5 days/week for 9 weeks. Horses were allocated randomly into fast or slow groups and exercised the same distance each day. Training distances were 1600 m in Weeks 0 and 1 up to 4000 m in Week 9. The fast group were trained at an intensity inducing a post training blood lactate of 4-8 mmol/l. This intensity was determined for each horse each week. The slow group trained at half the speed of the fast group (blood lactate < 2 mmol/l). Horses performed a standardised exercise test prior to (Week 0) and on Weeks 1, 2, 3, 4, 7 and 9 of training. HR, VO2, VCO2 and blood lactate concentration were recorded during the last 15 s of each step. Blood samples were collected at the end of each test for determination of red cell and plasma volume. Muscle biopsies were collected from the middle gluteal muscle before training and after 4 and 9 weeks training. Training intensity had few effects on the majority of variables measured and results for both groups are combined unless otherwise stated. Bodyweight was unaffected by training. Economy of locomotion decreased from 12.0 +/- 0.4 ml/kg bwt/m prior to training to 13.8 +/- 0.6 ml/kg bwt/m at the end of training in the fast group. Run time to fatigue was not affected by training intensity. VO2max increased from 120.3 +/- 4.8 to 144.7 +/- 3.5 ml/kg bwt/min with a significant correlation between run time and VO2max. Peak HR was 221.4 +/- 2.5 beats/min prior to training and 226.5 +/- 1.7 beats/min after the first 4 weeks of training. V200 and VLa4 increased in response to training. Similarly, VLa4 increased from 7.0 +/- 0.5 to 9.2 +/- 0.2 m/s with VLa4 correlated to VO2max. Plasma volume decreased from 29.1 +/- 1.7 to 25.8 +/- 0.9 l during the last 3 weeks of training. Blood volume, red cell volume and/or red cell volume/kg were unaffected by intensity or duration of training. The activity of CS in muscle increased in the first 5 weeks of training whereas HAD activity was not affected by intensity or duration of training.