Administration of 20 mg/kg cyclophosphamide to unilaterally nephrectomized rats for a period of two weeks significantly increased the ratio of kidney weight to total body weight. This rise is, however, accountable more by the decrease of total body weight than by weight increase of the kidney. The RNA-DNA ratio in the hyperttrophic kidney after cyclophosphamide did not differ significantly from that found in the hyperttrophic kidneys of controls. The results obtained show that in our experimental lay-out cyclophosphamide did not inhibit significantly the compensatory renal hypettrophy. After cyclophosphamide, the hypertrophic kidney produced urine with lower concentrations of endogenous creatinine, all osmotically active substances and potassium than found in controls.