Calcium has been shown to control the proliferation of various cells in vitro and in vivo. In this study we have attempted to modify compensatory renal growth by pharmacological interventions in mice who have undergone uninephrectomy. The effect of a calcium channel blocker verapamil was investigated. Unilateral nephrectomy of intact male mice produced the expected increase in weight of the remaining kidney by 67.5 ±8.1 %. This rise was accompanied by a proportional increase in RNA. In mice, cell hypertrophy was found to be a major factor in compensatory renal growth. Verapamil given in a i.p. dose of 1.0 or 2.0 mg/day/mouse attenuated the growth of the remaining kidney so that its weight rose by only 48.2 ±6 % and 28.2 ±4.4 %, respectively. In vivo administration of verapamil decreased the degree of compensatory renal growth and this growth inhibiting effect was directly proportional to the dose.