Glomerular filtration rate (GFR) was measured in 9 voluntary male kidney donors by scintigraphy. In the basal state, the mean GFR was 96.1 ± 22 ml/ min/1.73 m<sup>2</sup>. Four weeks after donor nephrectomy, the GFR of the remaining kidney had increased by 22.8 ± 20.7% (p < 0.05). A protein meal given at this time failed to significantly augment the GFR. The GFR of the grafted kidney increased by 59.7 ± 24.4% after 6 months (p < 0.01). Renal functional reserve was thus demonstrated in the remaining kidney and the allograft, though a further reserve could not be elicited in the remaining kidney.