The structural development of individual nephrons has been investigated in the kidneys of young rats by microdissection. Superficial, intercortical and juxtamedullary nephrons in Wistar-Konarovice rats aged 2 to 60 days have been followed. At two and five days of age the superficial nephrons were in an early stage of anatomical development. At this time intercortical and juxtamedullary nephrons were more developed, and basically resembled similar structures in adult animals with the exception that the loops of Henle did not have a thin ascending limb. At ten days of age all three types of nephrons were differentiated with the exception that the intercortical nephrons did not possess a thin ascending limb of Henle's loop. At selected stages of development there appears to be a direct relation between the length of the superficial nephrons and the weight of the kidney. Since individual segments of these nephrons do not grow at the same proportional rate, this relationship does not hold when compared to kidney weight.