The arterial and venous systems of the neck and heads of a dicephalic iniodymic monosomic cat are described. In the arterial system, an anomalous lingual branch of the right external carotid formed a large subhyoid arch, extending to the midline between both heads, giving off branches supplying the medial aspect of both heads and terminating in two medial internal carotid arteries each for the right or left head. This vessel was considered as an anomalous supernumerary common carotid artery. In the venous system, each lingual branch of linguofacial veins united and formed an anomalous venous arch. It received rostrally a long midline vein running between both heads and connecting with the medial internal jugular system. This vein received tributaries from the left and right heads and was considered as a supernumerary jugular vein. From these observations, and those of others in the literature, it can tentatively be suggested that, in congenital cephalic duplications in domestic mammals, the arterial blood supply for the medial aspect of both heads tends to come from the lingual branch of the external carotid artery, with an unexplained prevailing contribution of the right external carotid system. Two different venous patterns have been observed. In animal especies exhibiting a hyoid venous arch, such as carnivores, a midline supernumerary external jugular vein draining at the anomalous junction between lingual veins of both heads is formed, whilst in animals lacking a constant hyoid venous arch, such as ruminants, an asymmetrical supernumerary external jugular vein draining into the right "normal" external jugular vein has been repeatedly described.