The seasonal variation in physiological characteristics of elite male runners was studied. Five middle distance (mean age 21 yrs) and 5 long distance runners (23 yrs), all members of the Swedish national track and field team, participated in treadmill tests on 4 occasions over a period of one year: in January, in May, during the highly competitive summer period and the following January. The maximal oxygen uptake (VO2 max, ml X kg-1 X min-1) increased successively during the season and was significantly (p less than 0.01) higher during the summer than in the winter (74.2 to 77.4 ml X kg-1 X min-1). From the competitive summer period to the second winter the VO2 max (ml X kg-1 X min-1) showed a significant decrease. The absolute value of VO2 max (1/min) was not significantly changed during this one-year period, however. Running economy was evaluated from oxygen uptake determinations at 15 km/h (VO2 15) and 20 km/h (VO2 20). Slightly lower values of VO2 15 and VO2 20 were noted during the season, and after one year VO2 20 was significantly decreased. Such an improvement in running economy with time was also found in a larger group of elite runners (n = 16) when determined from an average of 7 treadmill tests. The running velocity corresponding to a blood lactate concentration of 4 mmol/l increased from January to the summer season. The blood lactate concentration after exhaustion (VO2 max test) increased significantly from January to May.