Polyamines (PAs) are ubiquitous polycationic metabolites in the eukaryotic and prokaryotic cells and are believed to be intimately involved in the regulation of DNA, RNA, and protein biosynthesis. However, the subcellular localization of PAs has not yet been fully elucidated in a variety of cell types. In the present study, a pre-embedding indirect immunoperoxidase approach was used to define the fine structural localization of PAs in the gastrointestinal tract of rat, which was fixed with glutaraldehyde and the monoclonal antibody ASPM-29 specific for spermine (Spm) and spermidine (Spd). Examination by a transmission electron microscopy showed that the peroxidase end products were commonly and predominantly localized in the free and attached ribosomes of the rough endoplasmic reticulum (rER) in the active protein- or peptide-secreting cells, and in rapidly proliferating cells including the gastric chief cells, mucous neck cells, and intestinal crypt cells. The nuclei, mitochondria, and secretory vesicles were devoid of PAs. Of note is the new finding that PAs are also located even on the small number of ribosomes in the cytoplasm of the parietal cells and of the villus-tip cells, because these were the cell types that were found to be almost PA-negative at the light microscopic level. These results seem to be completely consistent with those recently obtained for rat neurons. Thus, the present study generalized the subcellular localization of PAs on the ribosomes, and demonstrated that PAs are one of the components of biologically active ribosomes, possibly in any type of cell, that are closely involved in the translation processes of protein biosynthesis.