The cow breathes through a mask with a pneumatic fit into a spirometer from which the expired air is sucked through a long vertical glass tube against a spray of NaOH soln. A vacuum cleaner with a valve mechanism operated by the spirometer bell provides a proper ventilation rate without resistance to the cow''s breathing. Samples of the inspired and expired air are automatically and continuously collected for gas analysis. Usually the respiration trial lasts 3 hrs. after the injn. of a C14-labeled substance into a cow. BaCO3 is prepared from samples of the spray collected approx. at 2 min. intervals. The C14 content in these BaCO3 samples is measured with a Geiger counter.