Publisher Summary This chapter presents a study in which a series of experiments was done to map the presence of neuronal perikarya containing leucine-enkephalin (Leu-ENK) and methionine (Met)-enkephalin (Met-ENK) in the cat ventrolateral medulla (VLM). ENK immunoreactive neurons were identified in animals after the administration of colchicine into the cisterna magna and in animals that did not receive colchicine. Colchicine is known to disrupt axonal transport and result in the accumulation of peptides in neuronal perikarya. There is now a considerable amount of experimental evidence suggesting that neurons in the VLM are involved in the maintenance and reflex regulation of systemic arterial pressure. Recently it has been shown in the rat that the microinjections of an enkephalin analogue into the rostra1 VLM elicit decreases in arterial pressure, whereas microinjections into the caudal VLM elicit increases in arterial pressure. These data suggested that an enkephalinergic system in the VLM might be involved in controlling the circulation. In support of this suggestion, the rat VLM has been shown to contain neurons that are immunoreactive to Met-ENK and β -endorphin.