In a previous study, we described a new method [3] to measure Na reabsorption by each segment of the human nephron independently. Reabsorption was expressed as equivalent volumes of solute-free water (CH2O) generated by the loop of Henle (CH2O-HL) and by the distal tubule (CH2O-DT), and dissipated by back diffusion (BD) across collecting ducts (CH2O-BD). These data were obtained during maximal water diuresis (MWD). The present study was undertaken to calculate CH2O-HL by experiments performed during maximal antidiuresis (MA). For this purpose, a new theoretical approach was devised, described by algebraic equations which allowed calculations of segmental transport during MA alone, where only CH2O-HL could be calculated independently. The study was performed on 14 normal volunteers who were studied twice by clearance measurements, firstly during MWD and again during MA. In each experiment, clearance periods were performed during baseline conditions and during the administration of furosemide (0.7 mg/kg bolus injection followed by 0.06 mg/kg/min maintenance infusion). From the values measured during either condition, segmental reabsorption was calculated. During MWD, CH2O-HL averaged 19.4 + 10.4, during MA 20.4 + 8.0 ml/min/GFR X 100; p greater than 0.05. The paired measurements were significantly correlated (r = 0.80; p less than 0.01). These data demonstrate that CH2O-HL obtained with the original theory is a reproducible result that can be confirmed with independent measurements obtained during different experimental conditions. Thus, measurements of segmental Na transport in the human nephron are feasible and can contribute important informations on disease states.