Deletions of claudin-2 (Cldn2) and aquaporin1 (AQP1) reduce proximal fluid reabsorption (PFR) by about 30% and 50% respectively. Experiments were done to replicate these observations and to determine in AQP1/claudin-2 double knockout mice (DKO) if the effects of deletions of these established water pores are additive. PFR was determined in inactin/ketamine anesthetized mice by free flow micropuncture using single nephron I125-iothalamate (io) clearance. Animal means of PFR (% of GFR) derived from TF/Pio ratios in 12 mice in each of 4 groups (WT, Cldn2-/-, AQP1-/-, and DKO) were 45.8 ± 0.85 (51 tubules), 35.4 ± 1 (54 tubules; p<0.01 vs. WT), 36.8 ± 1 (63 tubules; p<0.05 vs. WT), and 33.9 ± 1.4 (69 tubules; p<0.01 vs. WT). Kidney and single nephron GFRs (SNGFR) were significantly reduced in all mutant strains . The direct relationship between PFR and SNGFR was maintained in mutant mice, but the slope of this relationship was reduced in the absence of Cldn2 and/or AQP1. Transtubular osmotic pressure differences were not different between WT and Cldn2-/- mice, but markedly increased in DKO. In conclusion, the deletion of Cldn2, AQP1, or of both Cldn2 and AQP1 reduces PFR by 22.7%, 19.6%, and 26% respectively. Our data are consistent with an up to 25% paracellular contribution to PFR. The reduced osmotic water permeability caused by absence of AQP1 augments luminal hypotonicity. Aided by a fall in filtered load the capacity of non AQP1-dependent transcellular reabsorption is sufficient to maintain PFR without AQP1 and claudin-2 at 75% of control.