Aerobic characteristics, oxygen debt and blood lactate were analysed in 20 male speed endurance athletes (400 m sprinters and 400 m hurdlers). The subjects were tested three times; at the beginning of March, at the end of May and at the end of August. Aerobic and anaerobic threshold and maximal oxygen uptake measured on the treadmill decreased (p < 0.05-0.01) from the second test occasion to the third one. The anaerobic work test on the treadmill was a constant load test at 5.56 m.s-1 with a slope of 4 degrees. The time to exhaustion increased (p < 0.05) from the first test occasion (112 +/- 17 s) to the second one (136 +/- 35 s) and did not change in the last test (135 +/- 25 s). Following the anaerobic work test oxygen debt was measured during 20 minutes. The highest total oxygen debt values (144 +/- 19 ml.kg-1) were observed in the second test occasion. Peak blood lactate following the anaerobic work increased (p < 0.05) from the first test occasion to the second one and remained at the same level during the next three competitive months. The good speed endurance athletes differed from the poor counterparts in time to exhaustion (p < 0.01), in 100 m record time (p < 0.01) and in maximal oxygen uptake (p < 0.05). Our results suggest that aerobic characteristics decrease during the competitive period in speed endurance athletes. The anaerobic performance capacity including work time and peak blood lactate is at high level in the competitive period.(ABSTRACT TRUNCATED AT 250 WORDS)