The purpose of this study was to assess influence of two isocaloric meals, the first one rich in fat (HF) (64 % fat (F), 19 % carbohydrates (CH), 17 % protein (P)), and the second rich in carbohydrates (HC) (18 % F, 64 % CH, 18 % P), on oxidative metabolism during rest and cycling. Ten healthy young male volunteers participated in two exercise tests. Two hours after the ingestion of either HF or HC breakfast oxidative metabolism was measured over a period of 30 minutes in resting supine position. Six minutes after the completion of rest, oxidative metabolism was measured during cycle ergometery, during which resistance was increased every 2 minutes for 0.5 kiloponds (kp) i.e. 27 Watt (W). We did not find any statistically significant differences in respiratory quotient (RQ), both in rest and during cycling, between HF and HC meal (p = 0.484 and p = 0.106). The lowest RQ during cycling in HC meal was significantly higher compared to HF meal (HC 0.85±0.10 vs. HF 0.79±0.07; p = 0.026), but we did not find any significant differences between the two conditions in the workload at the lowest RQ (p = 0.18). Heart rate and ratings of perceived exertion on a muscular level did not differ between the two conditions (p = 0,539 in p = 0,237), but a trend (p = 0,092) towards lower Borg ratings was observed for the respiratory level in the HC condition. No differences in highest fat oxidation velocity (Fatmax point) were observed between the two conditions (p = 0,442). In conclusion, the nutritional intervention with HF or HC meal influences the lowest RQ during cycling, but it does not influence the workload at the lowest RQ, nor the fat oxidation velocity.