Male and female marathon runners (n = 34) were studied in incremental and continuous running tests under both laboratory and field conditions. Aerobic capacity was determined based on the relationship between the lactate concentration and running velocity. We also analyzed the acid-base balance after the laboratory test of continuous running for 45 min. The individual running velocities in the incremental field test at given lactate concentrations were correlated with the marathon running velocities. Training workouts for six female runners were analyzed, and running speed during endurance training was compared with the lactate-velocity relationship in an incremental laboratory test. The main findings are summarized below. There is a very close relationship between the velocities determined at 2.5, 3, and 4 mmol/l in the incremental field test and the marathon running velocity (r = 0.88-0.99, P less than 0.001). The highest correlation between test and marathon velocities was found at a lactate concentration of 2.5 and 3.0 mmol/l. In field and laboratory running tests lasting 44 and 45 min at a speed chosen in accordance with the runner's current marathon time, lactate levels reached a steady state at approximately 3 mmol/l. A slight increase in blood lactate levels was compensated via respiratory mechanisms. In the continuous treadmill test (n = 8), we recorded the following changes after the first blood sample collection (i.e., 10 min) and post-exercise: blood lactate concentrations rose from 2.2 +/- 0.93 to 3.5 +/- 1.45 mmol/l; the negative base excess increased from -1.2 +/- 3.2 to -3.4 +/- 1.7 mval/l.(ABSTRACT TRUNCATED AT 250 WORDS)