Horseradish peroxidase conjugated to wheat germ agglutinin was used as the tracer to demonstrate the cells of origin of the main olfactory bulb (MOB) afferent fibers in the mouse. The neurons projecting to the olfactory bulb were counted by area to show the contribution of each brain area to the system of MOB afferents. The majority of neurons projecting to the MOB were found in the nucleus olfactorius anterior and the cortex cerebri, area pyriformis, being respectively 53.8% and 35.7% of the total number of labeled neurons counted. The average total number of neurons per mouse brain projecting to the MOB was about 146,600.