The relationship of maximal oxygen uptake (VO2 max) and resting oxygen uptake in two seasons of the year (winter/summer) was investigated in 119 young males. The subjects were conscripts, who were investigated at the calling-up time (winter: n = 58). The two groups were comparable with respect to their anthropometric data and smoking habits, as well as their occupational and leisure time activities. VO2 max was significantly (p less than 0.01) higher in the subjects investigated in the summer time (3.35 1/min) compared with those in the winter time (3.12 1/min). There was no difference in resting VO2 expressed relatively to body weight, whereas the resting VO2 was significantly (p less than 0.05) higher in winter than in summer, when it was expressed relatively to lean body mass and body surface area. The results suggest that seasonal variation in maximal aerobic power may occur in young males; this may in turn affect their response to physical training.