An exogeneous marker protein, horseradish peroxidase (HRP) was used to race peripheral autonomic pathways in adult guinea pigs and cats. Small doses of HRP were injected into various organs and after a brief survival period, HRP activity appeared in the perikarya of autonomic neurons that supplied each injection site. After injection of HRP into the anterior chamber of the eye, reaction product was detected in the postganglionic sympathetic neurons of the superior cervical sympathetic ganglion. In another experiment, HRP reaction product was found in the cell bodies of the preganglionic sympathetic neurons that supply the adrenal medulla. These were located in the lateral gray column of the spinal cord at T6 and T7 segmental levels. Reaction product appeared in intramural postganglionic parasympathetic neurons close to an injection site in the wall of the urinary bladder and in a similiar situation in Meissner's ganglia of the ileum. Following injection into the walls of the stomach and ileum, HRP labelled cells were detected in the nodose ganglion of the vagus and in preganglionic parasympathetic neurons in the dorsal motor nucleus of this nerve. After injection into the subepicardial tissue of the heart, reaction product appeared in the stellate ganglion and also in an upper thoracic dorsal root ganglion. These data suggest that HRP is taken up by peripheral autonomic nerves of all types, and then undergoes rapid retrograde axonal transport to the perikaryon. It appears, therefore, that HRP may be useful in tracing both motor and sensory peripheral autonomic pathways.