In the rabbit kidney accumulation of phenol red in cortex tissue is directly related to phenol red excretion. In histological preparations of rabbit cortex the major part of phenol red is localized to the pars recta of proximal tubules at low plasma concentrations of dye. The extra uptake of dye by the pars recta is abolished by administration of a high dose of probenecid and also by high plasma dye concentrations, when dye secretion is low relative to tubular reabsorption. Tissue accumulation of phenol red in the rat exhibits features similar to those in the rabbit. However, extra dye uptake in the pars recta is maintained after administration of probenecid, and disappears after intravenous injection of phenol red during ureteral occlusion to impede access of dye from tubule fluid to the luminal membrane. It is concluded that in the rabbit phenol red uptake by the pars recta probably is due to tubular secretion across the peritubular membrane, whereas in the rat extra uptake of dye by this segment is consistent with uptake at the luminal cell membrane.