Heart rate (fc) and post-competition blood lactate concentration ([La+]) were studied in seven male professional National Hunt jockeys over 30 races. The fc response for individual races followed a similar pattern for all subjects. The mean peak fc recorded during competition was 184 beats.min-1 (range 162-198 beats.min-1) with average fc during the races ranging from 136 to 188 beats.min-1. During consecutive races the recovery fc did not return to resting values. The mean [La+] was 7.1 mmol.l-1 (range 3.5-15.0 mmol.l-1). The conclusions of this study suggest that riding in National Hunt races is a physically demanding occupation. The muscular activity in this profession requires a high metabolic drive and produces a significant cardiorespiratory response.