Oligonucleotides may hybridize with high selectivity to an RNA sequence and can be used for the monitoring of gene expression or for its inhibition in experimental or therapeutic purposes. As part of the development of positron emission tomography (PET) methods, different lengths (30, 20, 12 and 6 mer) of antisense phosphorothioate oligonucleotides complementary to rat chromogranin A were labeled with [(76)Br] using a prosthetic group. The (76)Br-oligonucleotides were injected into rat's tail vein (1-2 MBq/rat), and the radioactivity distribution was analyzed after 20 h using whole body autoradiography or by measurement of organ radioactivity concentration. The whole body autoradiography showed different distribution depending on the oligonucleotide length. The organs with highest uptake changed from kidney cortex (with 6 or 12 mer), kidney cortex and liver (with 20 mer), to liver and spleen (with 30 mer). With 20 or 30 mer sequences, uptake could be observed in the adrenals. Kidneys and livers from rats receiving 20 mer or 30 mer (76)Br-oligonucleotides were analyzed with respect to subcellular distribution and DNA/RNA/protein fraction. 30%-45% of the radioactivity was found in the nuclear fraction. More than 80% of the radioactivity was recovered in the high molecular weight fraction (as proteins or oligonucleotides longer than 10 mer) using size exclusion (NAP 5) gelfiltration or cetylpyridinium bromide (CPB) precipitation. This work indicates the potential to perform kinetic whole body studies of (76)Br-oligonucleotides using PET.