Concentrations of spermidine and spermine in fifteen different tissues of male and female rats aged three months were determined, using amido black as the quantitative reagent, after paper electrophoretic separation of the amines. The changes in the concentrations of these polyamines in relation to age were followed by determinations from six tissues of newborn rats and of male rats aged 1, 3 and 9 months. At the age of three months the largest amounts of the polyamines were found in the liver and the thymus, 900 to 1600 mμmoles of spermidine and 700 to 900 mμmoles of spermine per g wet weight. No great sex differences were observed. In most tissues the molar concentration of spermidine was higher than that of spermine. The spermidine content in every tissue studied decreased with increasing age, the fall being most marked during the first month of life. The spermine content as a rule increased somewhat during the first month and then remained nearly constant or decreased slightly. Consequently, the molar ratio spermidine/spermine was highest immediately after birth, decreasing in the liver, for instance, from 4.52 to 0.84 during 9 months. In the brain, in contrast to the other tissues investigated, it increased from 1.31 to 2.08 during the first 9 months.