The functional and morphologic pattern of superficial nephron development was studied in guinea pigs ranging in age between 2 h and 38 days. Concomitent measurements of total kidney function and glomerular counts were also performed. Superficial nephron glomerular filtration rate was found to increase from 0.92 to 19.32 nl/min. The filtration rate of the entire kidney rose from 0.19 to 1.31 ml/min. During the first 15 days of life the average rate of increase in glomerular filtration rate per nephron (0.48 nl/min.day), obtained by dividing the increase in total kidney glomerular filtration rate by glomerular number, was more than twice the observed rate of increase in the superficial nephrons (0.21 nl/min.day). During the remainder of the first month, the increase in superficial nephron glomerular filtration rate (0.97 nl/min.day) was greater than the average increase for all nephrons (0.71 nl/min.day). Thus, the initial increase in total kidney glomerular filtration rate was primarily a consequence of the activity of the deep nephrons, whereas during the ensuing period the superficial nephrons appeared to be the sole contributors to the change in total kidney glomerular filtration rate.The increase in superficial nephron glomerular filtration rate was found to correlate closely with the increase in proximal tubular length. Functional glomerulotubular balance was maintained throughout the entire period of renal maturation.