The location of three radioactively labelled extracellular-space indicators ([ 3 H]methoxyinulin, [ 3 H]-D-mannitol and sodium [ 35 S]sulphate) in the superior cervical sympatmhetic and nodose ganglia of cats was studied using an autoradiographic technique. The technique was designed to eliminate movement of soluble, diffusible substances in tissues after excision, and to provide high autoradiographic resolution. Ganglia were rapidly excised after administration of the radioactive compounds in vivo , and frozen in liquid propane. Frozen sections were cut at -60°C at a thickness of 0.7-0. µ. The frozen sections were freeze-dried, dry-mounted on dried, photographic emulsion-coated microscope slides, and exposed at -15°C until development. The highest densities of silver grains in the autoradiographs were associated with regions of the tissue containing the greatest amounts of connective tissue. Lowest densities occurred beneath neurons and myelinated nerve fibres. The silver grain density beneath neuron perikarya was between 10% and 15% of that associated with plasma. Attempts were made to determine the source of these subneuronal silver grains. The results suggested that they could not be ascribed to the following: background; chemographs and pressure artefacts; spread of radiation from radioactive material outside the perikarya; and in vitro translocation of radioactive material into the neurons or into the emulsion beneath the neurons. It was concluded that the subneuronal grains reflected a small amount of intraneuronal penetration of the compounds in vivo . There was very little difference between inulin, mannitol and sulphate with regard to the proportion of intracellular activity. Except for this small amount of intracellular radioactivity, the findings accord with the view that inulin, mannitol and sulphate are confined predominantly to the extracellular fluid.