AbstractPenetration of macromolecules into living peripheral nerve was studiedin vivoandin vitrousing horse‐radish peroxidase. Peroxidase penetrates the epi‐ and perineurium and is sequestered in the Schwann cell body. It then is apparent in the incisures (clefts) of Schmidt‐Lanterman and the paranodal loops. After prolonged incubation the marker appears in the adaxonal Schwann cytoplasm and periaxonal space. It then is visualized inside the axon within vesicles and agranular endoplasmic reticulum both at the node and internodal regions. More rapid penetration occurs at the node of Ranvier than at the internodal r