In order to investigate the possibilities of studying the axoplasmic flow in vivo by means of radionuclide scanning 75Se-methionine was injected into the L6, L7 or S1 spinal ganglia of cats and the spread of the isotope recorded with a gamma camera. -2. After 4 hours the area of isotope distribution extended 1-2 cm cranially and caudally in the spinal cord and a further movement of 1-2 cm was noted during the following 2-5 days. -3. Scanning of removed specimens of the spinal cord, roots, ganglia and nerves confirmed the intramedullary uptake and in addition revealed an uptake in the dorsal root. Determination of the specific radioactivity showed the presence of the isotope in the ventral root and spinal nerve also, however, sufficient for scintigraphy. -4. Microautoradiographic studies with 3H-methionine demonstrated isotope uptake in axons of the dorsal roots and root entry zone of the spinal cord. Hence, the spread of the isotope was likely to have occurred by axoplasmic flow. This seems to provide a basis for further in vivo studies of this phenomenon.