Using a double immunostaining method, we demonstrated that type B monoamine oxidase (MAO-B) immunoreactivity was present in virtually all histamine (HA)-immunoreactive neurons in the posterior hypothalamus of the cat. Not all MAO-B-positive neurons, however, displayed HA immunoreactivity: a minor group of neurons immunoreactive for MAO-B alone was observed in the area dorsolateral to the caudal arcuate nucleus. The results suggest that the degradation of tele-methylhistamine might occur within the intraneuronal structures of histaminergic neurons.