An open-circuit indirect calorimetry system consisting of 4 climate-controlled respiration chambers for cattle has been constructed and validated. The system allows for the continuous monitoring of O(2), CO(2), and CH(4) concentrations in chamber air, and the simultaneous determination of feed and water intake, overall physical activity, position changes, standing and lying times, and animal behavior. For complete balance trials, feces, urine, and milk can be collected quantitatively. Most importantly, lactating cows can be milked in the chamber, and blood samples can be drawn from permanent catheters without disruption of the measurements. The investigator, on entering the chamber, wears a facemask connected to the ambient air during the whole milking process. Data are routed to a data acquisition system with appropriate data evaluation software developed in our research unit. Thus, dynamic changes of the above-named parameters during the course of the day or of longer time periods can be monitored. Such data are critical for understanding the complex regulation and interplay of feed intake, energy metabolism, climatic conditions, and milk production.