The post-hatching growth of the kidney of the chukar partridge, Alectoris chukar, was studied in birds aged 5, 24, 56, and 140 days. As body mass increased from 24 gm (5 d) to 479 gm (140 d), kidney mass increased from 0.16 to 1.6 gm. The number of nephrons in the kidney increased through age 56 d, but did not change thereafter. This increase was primarily a function of increasing development of the cortical regions; more than 90% of reptilian-type nephrons developed after hatching. The full complement of medullary cones was present at hatching. However, the lengths of the medullary cones and the number of loops of Henle of mammalian-type nephrons contained in each medullary cone increased through growth, so that approximately 65% of the mammalian-type nephrons developed after hatching. The chukar kidney contained 19% mammalian-type nephrons at age 5 d, but this was reduced to approximately 5% by 24 d, after which the proportions of mammalian-and reptilian-type nephrons did not change. This study demonstrates that metanephric kidney growth in birds begins in the deepest regions of the kidney and then extends peripherally but also that mammalian-type nephrons continue to develop throughout kidney growth, presumably from pre-existing reptilian-type nephrons.