We studied absorption, distribution, metabolism, and excretion of polyamines (putrescine, spermidine, and spermine) in the gastrointestinal tract using ¹⁵N-labeled polyamines as tracers and ionspray ionization mass spectrometry (IS-MS). The relatively simple protocol using rats bearing solid tumors provided useful information. Three ¹⁵N-labeled polyamines that were simultaneously administered were absorbed equally from gastrointestinal tract, and distributed within tissues at various concentrations. The uptake of ¹⁵N-spermidine seemed preferential to that of ¹⁵N-spermine since the concentrations of ¹⁵N-spermidine in the liver and tumors were higher, whereas those of ¹⁵N-spermine were higher in the kidney, probably due to the excretion of excess extracellular spermine. Most of the absorbed ¹⁵N-putrescine seemed to be lost, suggesting blood and tissue diamine oxidase degradation. Concentrations of ¹⁵N-spermidine and ¹⁵N-spermine in the tumor were low. We also describe the findings from two rats that were administered with ¹⁵N-spermine. The tissue concentrations of ¹⁵N-spermine were unusually high, and significant levels of ¹⁵N-spermidine were derived from ¹⁵N-spermine in these animals.