1. Free water clearances were measured during infusion of hypotonic saline, glucose, urea, and mannitol in Brattleboro rats. For each solute the free water clearances were plotted using either V or (C(H2O) + C(Na)) as the distal tubular delivery term.2. In all cases the use of (C(H2O) + C(Na)) as distal delivery term yielded a steeper relationship than when V was used. There were no significant differences in the C(H2O) to V relationship when saline, glucose and mannitol was the solute infused. Urea, however, resulted in a curve with a slope significantly less than that for the other solutes.3. When C(H2O) was plotted against (C(H2O) + C(Na)) there was still no significant difference between the slopes of the curves during saline or mannitol infusion. Use of this delivery term, however, resulted in a slope during glucose infusion which was significantly greater than that during saline or mannitol infusion. The slope for urea infusion remained lower than that for any other solute.4. Regardless of the delivery term used, there was no significant difference in the slopes of the curves for awake Wistar and awake Brattleboro rats during mannitol infusion. This indicates that the awake rat is a suitable model for free water clearance studies.5. The results indicate that NaCl and mannitol are both adequate for free water clearance and that (C(H2O) + C(Na)) is a better index of distal delivery than V.