High ability to perform strenuous exercise of short duration is accompanied by a large lactate formation in the exercising muscles, but the disturbances in extracellular acid-base and electrolyte balance might be attenuated compared to subjects with less ability to perform intense exercise. To study this, oxygen deficit, changes in arterial blood acid-base status and plasma electrolytes were studied in six-endurance trained (ET) and six sprint-trained (ST) subjects who exercised on a treadmill at a speed which led to exhaustion within 1 min. During exercise the ET and ST subjects developed an oxygen deficit of 41 and 56 ml oxygen units kg-1 respectively, whereas peak blood lactate concentration post exercise averaged 12.5 and 16.7 mmol l-1. Blood pH followed lactate concentration closely, reaching nadir values of 7.175 and 7.065 for ET and ST subjects respectively. Respiratory compensation and changes in blood bicarbonate and standard base deficit (SBD) concentrations for a given lactate concentration were the same for the two groups, amounting to a change in PCO2 of 0.12 kPa, in bicarbonate concentration of 1.09 mmol l-1 and in SBD of 1.44 mmol l-1 mM-1 change in blood lactate concentration. During exercise the increase in haematocrit, from to 43 to 45% for the ET subjects and from 46 to 50% for the ST subjects, was accompanied by almost parallel relative changes in plasma chloride and sodium concentrations. Whereas haematocrit continued to increase post exercise and followed blood lactate concentration closely, plasma sodium and chloride concentrations decreased to pre-exercise values within 9 min of recovery. The anion gap increased significantly more than blood lactate concentration. Thus, ST subjects were capable of accumulating more lactate in blood compared with ET subjects, but at the expense of a lower pH, since the buffer capacity seemed to be the same for the two groups. The acidosis, which was larger than could be accounted for by lactic acid, was associated with an inexplicably large anion gap.