The apparent biological half-lives of spermidine and spermine in mouse brain and other organs were determined by measurement of the specific radioactivities of these compounds over long periods of time. The endogenous polyamine pools were labeled by repeated intraperitoneal injection of [1,4-14C]putrescine.2HCl, [2-14C]D,L-methionine, [2-3H]L-methionine and S-adenosyl-[2-3H]L-methionine. Repeated injection were given to ensure labeling of both fast and slow polyamine pools. It was shown that the two parts of the polyamine molecules which derive from ornithine and methionine have significantly different life spans, especially in the brain. Actual turnover rates of polyamines could not be determined because of the active interconversion between spermine and spermidine, and between spermidine and putrescine. The observed reutilization of putrescine originating from spermidine degradation for spermidine biosynthesis, and the analogous reutilization of spermidine in spermine biosyntehsis is discussed with respect to its physiological significance and its relationship to cellular organization.