The permanence of the early phases of compensatory renal hypertrophy was tested by interrupting vascular parabiosis between an anephric rat and a normal rat after 48 h. At the time of interruption, the weights and the ratio of RNA/DNA of the kidneys were the same as those of the remaining kidney in a single rat subjected to unilateral nephrectomy, previously reported. Within 12 h after parabiosis was stopped, renal mass and nucleic acid concentrations returned to normal. Compensatory hypertrophy could be produced again by unilateral nephrectomy. Regression of the early phase compensatory hypertrophy appears to be faster than muscular atrophy produced by disuse or denervation. Compensatory hypertrophy can be activated at least twice.