The aim of this study was to investigate the physical demands of international rugby union. Five games in the 1989-90 Five Nations Championship were analysed using video-recordings of live television transmissions. When the ball was in open play, the average running pace of players central to the action ranged from 5 to 8 m s-1. This together with scrum, lineout, ruck and maul was classified as high-intensity exercise. The density of work was measured by timing the work:rest ratios (W:RRs) throughout each game. The mean duration of the work periods was 19 s and the most frequent W:RRs were in the range of 1:1 to 1:1.9. On average, a scrum, lineout, ruck or maul occurred every 33 s. The ball was in play for an average of 29 min during a scheduled time of play of 80 min. To complement the time-motion analysis, blood samples were taken from six players throughout a first-class game. The highest measured blood lactate (BLa) concentrations for each individual ranged from 5.8 to 9.8 mM. Running speed, duration, BLa levels, physical confrontation and, most particularly, the density of work as illustrated by the W:RRs indicate that the game places greater demands on anaerobic glycolysis than previously reported. This has implications for the physical conditioning of rugby union players.