The aim of this study was to examine heart rate, blood lactate concentration and estimated energy expenditure during a competitive rugby league match. Seventeen well-trained rugby league players (age, 23.9 +/- 4.1 years; VO2max, 57.9 +/- 3.6 ml.kg-1.min-1; height, 1.82 +/- 0.06 m; body mass, 90.2 +/- 9.6 kg; mean +/- s) participated in the study. Heart rate was recorded continuously throughout the match using Polar Vantage NV recordable heart rate monitors. Blood lactate samples (n = 102) were taken before the match, after the warm-up, at random stoppages in play, at half time and immediately after the match. Estimated energy expenditure during the match was calculated from the heart rate-VO2 relationship determined in laboratory tests. The mean team heart rate (n= 15) was not significantly different between halves (167 +/- 9 vs 165 +/- 11 beats.min-1). Mean match intensity was 81.1 +/- 5.8 % VO2max. Mean match blood lactate concentration was 7.2 +/- 2.5 mmol-1-1, with concentrations for the first half (8.4 +/- 1.8 mmol.1-1) being significantly higher than those for the second half (5.9 +/- 2.5 mmol.1-1) (P < 0.05). Energy expenditure was approximately 7.9 MJ. These results demonstrate that semi-professional rugby league is a highly aerobic game with a considerable anaerobic component requiring high lactate tolerance. Training programmes should reflect these demands placed on players during competitive match-play.