This study explores the role of brush border (luminal) membranes in p-aminohippurate (PAH) transport by the rabbit kidney. Under control conditions PAH appears in urine at the same rate as simultaneously injected inulin; no evidence for a secretory delay was found. After suppression of secretion by limiting concentrations of Benemid (probenecid), urinary PAH is largely derived from glomerular filtrate: its tubular transit time exceeds that of inulin. Excess Benemid abolishes this delay. Filtered PAH on its way to excretion appears to be permitted by a mechanism sensitive to high concentrations of Benemid to pass through an extraluminal volume equivalent to that of the proximal tubule cells. These results strengthen the previously suggested model for tubular PAH transport, and provide justification for use of glomerulus-to-ureter transit times in localizing tubular transport mechanisms.