Retention equations of Cs in single internal organs in the human body were derived by compartmental analysis. A 5-compartment mammillary system was adopted as the model for Cs metabolism, because it could explain the experimental data on the retention of Cs in the single internal organs in rats. The model for humans was derived from the rat model by modifying the rate constants of it to fit human data but leaving the type of the model unchanged. The retention equations in the muscle, testes, liver and bone in the human body were derived.