SummaryDespite the progresses accomplished during the past twenty years, knowledge of the mechanism of water and sodium renal excretion remains very incomplete. The uncertainties concern more particularly the physiologic process(es) currently grouped under the«third factor»heading. The role of the proximal tubule in the genesis of certain urinary anomalies becomes clearer.«Proximal urine»and«proximal tubular acidosis»concepts are currently used in present medical jargon. After renal transplantation, urine may be precociously and transiently hypotonic despite plasma hypertonicity, and high urinary bicarbonate concentration may coexist with metabolic acidosis. The role played by profuse polyuria of osmotic type in these anomalies, with decreased proximal tubular reabsorption of water and sodium, is illustrated and discussed.