1. The renal excretion of phenol red and other phenolsulphophthalein dyes (bromophenol blue and bromothymol blue) was studied in clearance experiments on anaesthetized rabbits. 2. Net tubular excretion of phenol red reached a maximal value of 8 mumole/min at a plasma concentration of ultrafiltrable dye of about 0.1 mM and was decreased at higher plasma concentrations. Decreases in net tubular excretion at high plasma concentrations were also obtained for bromophenol blue and bromothymol blue suggesting tubular reabsorption in addition to tubular secretion of the dye. Conclusive evidence for reabsorption was provided by administration of probenecid which caused a fall in the excretion of the dyes below that filtered by the glomeruli. 3. Tubular reabsorption of phenol red during probenecid administration appeared to be proportional to the glomerular load and was increased under experimental conditions leading to a decrease of urinary pH. Experiments involving efflux of phenol red from liposomes gave no evidence of a significant role of transmembrane passage by non-ionic diffusion. It is suggested that the pH dependence of the reabsorptive process is the result of preferential reabsorption of the acid as compared to the basic form of the indicator dye across a hydrophilic pathway in the transporting membranes. 4. Clearance ratio of phenol red to that of p-aminohippurate at low plasma concentrations was about 0.3. They low degree of extraction of phenol red from renal plasma is attributed both to tubular reabsorption and binding of the dye by plasma proteins.