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The intracellular distribution of horeseradish peroxidase (HRP) transported intraaxonally from the caudoputaminal complex to the substantia nigra has been examined with the electron microscope. The reciprocal axonal connections between the caudoputamen and the substantia nigra permitted observation not only of HRP transported retrogradely from axons and axon terminals in the caudoputamen to the cell bodies of origin in the pars compacta of the substantia nigra, but also provided information suggesting that HRP may be transported anterogradely by neurons of the caudoputamen to their terminals, which are especially numerous in the pars reticulata of the substantia nigra. Special attention was focused on observations which might elucidate the manner in which exogenous proteins are compartmentalized and transported intracellularly. It is suggested that the agranular reitculum is involved in the retrograde transport of proteins which are pinocytosed near the axon terminal and ultimately reach lysosomes in the perikaryon. A possible anterograde movement of HRP may also involve the agranular reticulum. The implications such findings have on the use of HRP in neuroanatomical tracing techniques are also discussed.