3 3 14 14 3 3 3 3 3 1 5 3 3 3 14 3 H labeled bradykinin ([ H]BKN) and C labeled inulin ([ C]In) were simultaneously microinfused into proximal or distal tubules in Inactin anesthetized rats, and urinary excretion and tubular transit times were measured. In other experiments higher doses of [ H]BKN were microinfused and intact peptide and its metabolites identified and quantified by two dimensional peptide mapping. The site of infusion was identified by neoprene injection and microdissection. Urinary recovery of H label was 24.1% when proximal tubules were infused and 98.0% when distal tubules were infused. For proximal tubules, 85% of H activity recovered from urine consisted of metabolites (81% [ H]Pro and 4% [ H]Arg Phe ) and 15% was intact BKN. Urinary recoveries of [ H]BKN and metabolites were unrelated to tubular length. With distal tubules all H activity appeared as intact BKN. Excretion curves of simultaneously infused [ H]BKN and [ C]In showed no marked differences in configuration for either proximal or distal tubules. The authors suggested that removal of [ H]BKN by proximal tubular cells occurs by rapid enzymatic cleavage at the luminal surface with reabsorption of most of the products and excretion of the remainder.