Fluoro-Gold injected either subcutaneously or intravascularly results in extensive retrograde labelling of CNS neurones having axons terminating outside the blood-brain barrier.

Subcutaneous or intravascular injections of the dye Fluoro-Gold resulted in widespread labelling of neurones with a distribution and appearance consistent with their being motoneurones, preganglionic autonomic neurones and hypothalamic neurones projecting to the pituitary. Peripheral nerve ligation prevented labelling of neurones whose axons travelled in that peripheral nerve. These observations suggest that Fluoro-Gold diffuses extensively in the body tissues but does not cross the blood-brain barrier and that it is taken up by peripheral terminals and transported retrogradely by motoneurones.