This essay looks at the historical significance of an APS classic paper (http://www.the-aps.org/publications/classics/) that is freely available online: Falk G. Maturation of renal function in infant rats. Am J Physiol 181: 157-170, 1955 (http://ajplegacy.physiology.org/cgi/reprint/181/1/157). Infants drink mother's milk, which is a hypotonic solution, and thus must be able to excrete a hypotonic urine from birth. In addition, under conditions of stress the neonate and infant must have some protection against volume depletion with conservation of salt and excretion of hypertonic urine. In Dr. Gertrude Falk's classic paper entitled "Maturation of Renal Function in Infant Rats" published in 1955, she examined the maturation of renal function during postnatal development in rats. She examined several of the basic functions of the kidney in adult and neonatal rats and compared the glomerular filtration rate, rates of excretion of a water load and salt load and the ability to concentrate urine when deprived of food and water for 24 hours. This study set the standard for how to carefully characterize immature renal function and the response of the neonate to environmental stress.