This study tries to find a way of improving the effectiveness of the training schedule for swimmers. We regularly measured the level of blood lactate during training in order to estimate as precisely as possible the anaerobic threshold in swimming. As is already known sportsmen improve more rapidly their physical capacity (especially endurance) when they swim at a threshold speed corresponding to 4 Mmol of lactate. We used the following parameters: blood lactate concentration in relation to swimming time, and blood lactate accumulation in relation to covered distance for successive days of training at the same swimming speed. These two complementary procedures allowed us to follow the state of training of each athlete using simple graphs and to better advise him so that he may attain his threshold swimming speed. We have often noticed an improvement of the physical condition: either as an increase in swimming speed for the same accumulation of lactate, or as a decrease in the level of blood lactate for a same swimming speed. Measurements of blood lactate accumulation in relation to covered distance, on the condition that the speeds are more or less the same, allow a more precise estimate of the athletes' performances.