No neurons in the laterodorsal tegmental nucleus (LDTg) show monoamine oxidase (MAO) activity in the rat or monkey. However, in our recent study, many LDTg neurons with MAO type B (MAOB)-activity were found in MAOA-deficient mice that were derived from C3H mouse line. In the present study, LDTg neurons with MAOB-activity were found not only in normal C3H mouse but also in BALB/C and C57BL/6 mouse lines: MAO histochemistry revealed LDTg neurons with MAO-activity even after pharmacological suppression of MAOA-activity with clorgyline, a specific MAOA inhibitor, but not after pharmacological suppression of MAOB-activity with deprenyl, a specific MAOB inhibitor. LDTg neurons with MAOB-activity also showed NADPH-diaphorase-activity, a marker of cholinergic neurons.