Rates of lactic acid accumulation during constant-speed swimming have been investigated by Treffeneet al. At speeds below a critical submaximal speed Vmax the lactic acid accumulation was less than 0·55 m moll−1 min. When Vmax speed (the maximum speed at which maximum heart rate and therefore maximum oxygen uptake occurs) is exceeded the rate of lactate accumulation exceeds 1·0 m moll−1 min−1. The rate increases rapidly as speed increases above Vmax. The implications of these measurements on performance of athletes above and below the Vmax speed is indicated and can lead to better understanding of elite performance in sport.