Twenty well-trained runners (˙VO2max 4.6 ± 0.5 L·min-1) were age and ability matched and assigned to either a cross training (CT) or run only group (RT). All subjects maintained normal running distance and intensity for 6 wk and reported for three additional training sessions per week. These workouts were performed outdoors on a 400-m track or measured road course (RT) or on a bicycle ergometer (CT). The sessions were as follows: (work·rest-1 ratio = 1): 5 × 5 min at >95% ˙VO2max/peak (Monday), 50-60 min at 70%˙VO2max/peak (Wednesday), and 3 × 2.5 min at >105%˙VO2max/peak, plus 6 × 1.25 min at >115%˙VO2max/peak (Friday). Subjects were tested before (PRE), after 3 wk (MID), and after 6 wk (POST) of intensified training. Blood samples were obtained from RT, CT, and ten controls (CON) at each time point (0600 h). Runners also completed a 10-min submaximal run at the same absolute intensity(velocity to elicit 75% of initial ˙VO2max) during which heart rate, RPE, and ˙VO2 were measured. Each runner then completed a simulated 5-km race (time trial) on a treadmill. Total testosterone (TT), free testosterone (FT), cortisol (C), and creatine kinase activity (CK) were determined. Running economy was similar between RT and CT; however, RPE decreased significantly at MID and POST compared with that at PRE (P< 0.05; time effect). There were no significant differences among groups for TT, FT, or CK, but C was significantly lower in CON than in RT and CT. Performance was significantly faster (P < 0.05; time effect) in the 5-km race at MID (1076.1 ± 81.4 s) and POST (1068.6 ± 83.9) compared with PRE (1096.6 ± 79.5) but was not different between CT and RT. In conclusion, RT and CT responded similarly to 6 wk of increased training, and both groups improved 5-km performance to a similar extent.