The projection from the cortex to the substantia nigra (SN) in the rat was studied using the biotinylated dextran amine (BDA) anterograde tracing method. Injections of BDA into the prefrontal cortex consistently yielded labeling of the thin nerve fibers with small boutons in the SN pars compacta (SNc) and the pars reticulata (SNr). The cortico-nigral projection had a loose medio-lateral and rostro-caudal arrangement. Injections of BDA into the precentral lateral, the parietal, the temporal, and the occipital cortices resulted in sparse or no labeling of boutons in the SN. The density of boutons in the most heavily labeled region of the SN was 0.23/10(3) micron3. For comparison, similar measurement was performed in the cortico-striatal projection. The density of the boutons in the the neostriatum was 4.9/10(3) micron3. This study indicates that the entire prefrontal cortex projects to the SNc and SNr and that the terminal density of this projection is much less than the cortico-striatal projection.