Under controlled conditions of environmental light and darkness of 12:12 h, the concentrations of noradrenaline, dopamine, and serotonin in six brain regions (i.e., cortex, hippocampus, striatum, midbrain including hypothalamus, pons-medulla oblongata, and cerebellum) of the male Mongolian gerbil, an animal widely used in stroke research, were measured fluorometrically at different times of the day. The significant daily fluctuations in these monoamine concentrations were observed in cortex, striatum, hippocampus, midbrain including hypothalamus, and pons-medulla oblongata. Thus, diurnal variations have to be taken into account when studying the effects of ischemia on the change of monoamine concentrations in these brain areas of the gerbil.