Two catabolic pathways exist for spermidine and spermine. One is responsible for the interconversion of the polyamines, a physiological intracellular event. The first and probably rate limiting step of the polyamine interconversion pathway is acetylation in the N1-position by a cytosolic enzyme. The reaction products N1-acetylspermine and N1-acetylspermidine are substrates of the cytoplasmic polyamine oxidase. This enzyme transforms the N1-acetylpolyamines into spermidine and putrescine respectively. N1-Acetylspermidine is at the same time a major urinary excretion product. The factors which control the rates of N1-acetylspermidine degradation by polyamine oxidase versus its elimination via transport are not known. The second catabolic pathway forms putreanine from spermidine and N8-(2-carboxyethyl)-spermidine and spermic acid from spermine. It is catalyzed by the well known serum spermine oxidase. The second step in this reaction sequence, the dehydrogenation of the aldehydes formed by the serum spermine oxidase occurs intracellularly and is catalyzed either by specific or non-specific aldehyde dehydrogenases. The function of this "two compartment reaction sequence" is most probably to protect tissues from extracellular or exogenous (alimentary) polyamines. Its end-products appear to be physiologically indifferent urinary excretion products. Both catabolic pathways may have marked effects on the urinary polyamine pattern. Drugs as well as a variety of physiological and pathological states may influence polyamine catabolism and elimination at various levels, and may cause characteristic alterations in the urinary excretion of free and conjugated polyamines and of the amino acids deriving from the polyamines.