Gas exchange measurements by means of indirect calorimetry can be used to calculate quantitative substrate oxidation. The results represent average net oxidation values (substrate disappearance rate), but they cannot describe the dynamics of the oxidation processes. Breath test measurements with substrates labelled with C-13 provide an attractive tool to describe the dynamics of oxidation processes, and may in combination with indirect calorimetry refine estimation of substrate oxidation. The objective of our investigation was to estimate oxidation of 1-C-13 labelled leucine in mink in response to feeding and fasting. Twelve 1-year-old male mink (Mustela vison) were measured in each five consecutive periods by means of indirect calorimetry and simultaneous breath test. In Periods 1, 3 and 5, each lasting 3 days, the animals were fed ad libitum and Periods 2 and 4 were fasting periods, each of 48 h. In Periods 1 and 5 all animals were fed a diet with a high quality fish meal (FISH; n=12), while in Period 3 half of the animals received the FISH diet (n=6) and the other half a diet with soy protein concentrate (SOY; n=6) as main protein source. An intraperitoneal injection of 1-C-13-leucine was given before measurements started and expired air was then sucked out of the respiration chamber and collected into breath bags at frequent intervals until 5.5 h after the start of measurements. The ratio of C-13/C-12 was measured by means of an IRIS infrared analyser and results are reported in terms of delta over baseline (DOB) values. There was no significant effect of dietary treatment group, but the interaction between treatment group and sampling time was significant (P=0.02), peak DOE values being recorded 70-135 min after injection in FISH animals, and 70-120 min in SOY animals. The effect of period was significant (P=0.03), values generally being lower during fasting, indicating a lower rate of leucine oxidation. It was concluded that the present results clearly demonstrate differences in rate of oxidation of leucine between fed and fasted animals.