Castration and testosterone replacement therapy served as a model to assess anabolic activity in the rat. The present study was undertaken to examine more closely whether changes in cellular anabolic activity are reflected in subsequent alterations of urinary N1 -acetylspermidine excretion. A sensitive fluorometric assay was employed which utilized the dansylation reaction . The dansylated derivatives were quantitated by normal phase high pressure liquid chromatography. This assay system exhibited a range of linearity from 0.1 to 10 nanomoles which encompassed the physiological variability observed during analysis of urinary N1-acetyl spermidine. The normal level of N1-acetylspermidine excreted in rat urine was found to be 803.1 ± 208.9 nanomoles per day (mean ± 1 S.D.). In addition both N8-acetylspermidine and acetyl putrescine were detected, but not quantitated. Due to the high variability of the polyamine excretion, no statistically significant difference could be detected between castrated animals, castrated animals receiving testosterone, and sham operated animals. Still some trends were noted which were in agreement with the hypothesis that urinary excretion of polyamines may reflect the anabolic state of the animal. Further studies will be required to determine whether or not a correlation between anabolic activity and urinary excretion of acetylated polyamines exists.