SummaryChanges in glomerular number and perfusion during maturation of the guinea pig were compared with the response to uninephrectomy at birth or 84 days of age. From birth to adulthood, there was a 21% increase in number of glomeruli identified by the presence of India ink previously injectedin vivo. For animals uninephrectomized at birth, a similar increase occurred several wk earlier than in sham-operated littermates; however, uninephrectomy in adulthood resulted in no further increment. Although the number of glomeruli in the left kidney varied by 25% within each group, the difference in number between right and left kidneys of individual animals averaged only 4%.Additional glomeruli not identified by India ink were subsequently revealed by application of Wright's stain. These comprised 13–17% of the glomeruli containing India ink in 1 and 22-day-old sham groups, but only 2% in 22-day-old uninephrectomized or adult animals. Approximately 3/4 of all glomeruli indentified by India ink were present in the outer cortex of 22-day-old uninephrectomized and sham-operated guinea pigs; the cortical distribution of Wright-stained glomeruli in the latter group was similar.Paraffin sections of kidneys revealed scattered glomeruli not containing India ink in 1-day-old and 22-day-old sham guinea pigs, but almost none in 22-day-old uninephrectomized animals. There were no glomeruli in an early stage of formation in any of the sections examined.It is concluded that during development, an increase in number of glomeruli identified by India ink represents nephrons which are underperfused in the neonate but are completely perfused at maturity. This process is accelerated by uninephrectomy in early development, but is unaffected by uninephrectomy in adulthood, at which time glomerular perfusion is virtually homogeneous.