Female C3H/Helms mice were reared from 4 weeks of age during the lifetime in wire-mesh cages with nozzle for automatic water supply on a battery with an automatic flushing system. The evaluation of this system was made in comparison with the conventional rearing system with water bottles, alumite's cages and changes of the bedding. The animals maintained at the automatic rearing system showed no significant difference as compared with the conventional one in weight gains, food intake and water consumption during the period from 4 to 25 weeks, and likewise in sequential change of mortality, incidence of mammary tumors, and survival time after the tumors began to develop.