The uptake of phenolsulfonphthalein (PSP) and of paraaminohippuric acid (PAH) by cortical slices of the rabbit kidney was investigated while varying the composition of medium. The overall uptake of these substances displayed typical active transport characteristics and was significantly enhanced in presence of acetate. When the phosphate buffer was used the optimal pH was 7.4 for both substances. However, when the tris-buffer was used the optimal pH was 7.4 for PSP and 8.3 for PAH. Removal of Na⁺ from the medium resulted in a significant reduction in the uptake. Similar results, though lesser in magnitude, were obtained when either K⁺ or Ca⁺⁺ was removed from the medium. However, there was no additive effect when K⁺ and/or Ca⁺⁺ were additionally removed from the Na⁺-free medium. The presence of NH₄⁺ greatly reduced while Li⁺ and Mg⁺⁺ moderately reduced the uptake of both substances. However, choline had no effect. In substrate-leached slices, acetate greatly enhance the uptake of organic acids; but this action was not demonstrable in absence of Na⁺, K⁺ or Ca⁺⁺.