The kidneys of fetal rats from uninephrectomized mothers were investigated morphometrically on days 18, 20, and 22 of gestation. Maternal uninephrectomy was performed on day 5. On days 20 and 22, the proximal tubular cell height in fetuses from uninephrectomized mothers was significantly lower than that in fetuses from sham-operated mothers. The luminal diameter, length and total volume of the proximal tubule were significantly larger in fetuses from uninephrectomized mothers than in those from sham-operated mothers. The total glomerular volume of mature type in fetuses from uninephrectomized mothers was significantly larger than that in fetuses from sham-operated mothers. On day 18, maternal uninephrectomy caused no significant difference in any kind of parameters used for the proximal tubule, but induced an increase in the total glomerular volume of immature type. These results suggest that maternal uninephrectomy stimulates morphological development of the proximal tubule in the fetal kidney.