The metabolic fate of stable isotopically labeled polyamines was investigated after their first and second intraperitoneal injection in rats. Using gas chromatographic and mass fragmentographic analyses of acid-hydrolyzed 24-h urines, some aspects of the polyamine metabolism could be elucidated. After the injections with hexadeutero-1,3-diaminopropane, obly labeled 1,3-diaminopropane was recovered from the urine samples. The rat injected with tetradeuteroputrescine excreted labeled putrescine excreted labeled putrescine, γ-amino-n-butyric acid, 2-hydroxyputrescine and spermidine, while the urine samples of the rat after the injections with tetradeuterocadaverine contained labeled cadaverine and δ-aminovaleric acid. The injections of hexadeuterospermidine led to the appearance of labeled spermidine, isoputreanine, putreanine, N-(2-carboxyethyl)-4-amino-n-butyric acid, putrescine, γ-amino-n-butyric acid, 1,3-diaminopropane, β-alanine and spermine. After the injections with octadeuterospermine, labeled spermine, N-(3-aminopropyl)-N′-(2-carboxyethyl)-1,4-diaminobutane, N,N′-bis(2-carboxyethyl)-1,4-diaminobutane, spermidine, isoputreanine, putreanine, N-(2-carboxyethyl)-4-amino-n-butyric acid, putrescine, 1,3-diaminopropane, β-alanine, 2-hydroxyputrescine and possibly γ-amino-n-butyric acid were recovered. Clear differences between the metabolism after the first and second injection were noted for putrescine, spermidine and spermine, which is suggestive for enzyme induction and/or the existence of salvage pathways.