By utilizing ordinary laboratory equipment and a spherical feces-urine separator, a simple, inexpensive metabolism cage for small mammals can be constructed. A hardware cloth animal cage over a cylindrical battery jar containing a spherical feces-urine separator affords the following advantages not commonly found in commercial metabolism cages: 1) complete separation of feces and urine through minimal contact, 2) minimal evaporation of urine due to proximity of storage vessel and lack of exogenous air currents, and 3) extremely low cost of less than five dollars. The metabolism cage is designed to allow measurement of fluid intake, and to separate and collect feces and urine for numerous qualitative and quantitative determinations. In addition, the metabolism cage permits observation of the animal, feces, and urine at all times, is readily cleaned or sterilized, and is easily fashioned from common laboratory equipment.