Thirty specimens of Hydrochoerus hydrochaeris were injected with neoprene latex in order to study the distribution of vessels which compose the vertebro-basilar system and its derived branches in the encephalon. The right and left vertebral arteries anastomosed on the ventral surface of the encephalon to form the basilar artery, which gave rise to the caudal cerebellar (left and right, double and single) and middle cerebellar (both antimeres, single and double) as collaterals. The basilar artery splits into right and left terminal branches which gave rise to a series of arteries which included the rostral cerebellar arteries (right antimere, double and single; left antimere, double, single and triple), the caudal cerebral arteries (double, single and triple in both antimeres), the internal ophthalmic arteries (single in both antimeres), and the middle cerebral artery (single in the right antimere in all specimens and single and double in the left antimere). The ending branches of the terminal branches of the basilar artery were the rostral cerebral arteries (single or absent in both antimeres) with a rostral communicating artery (single or absent) between them. The arterial circle was derived solely from the vertebro-basilar system, since internal carotid arteries were absent in adults. The circle was closed rostrally in 90% of the specimens and open in 10%. In all cases, there was anastomosis of the circle with maxillary artery.