Three different pairs of double retrograde axonal tracers have been used to study the distribution of axon collaterals from individual cells in the olfactory bulb and the nucleus of the horizontal limb of the diagonal band: (1) horseradish peroxidase (HRP) and tritiated apo-HRP (3H-HRP), (2) HRP and 125I-wheat germ agglutinin (I-WGA), and (3) the fluorochromes true blue (TB) and bisbenzimide (BB) or nuclear yellow (NY). With each combination of tracers, paired injections were made into different parts of the olfactory system, and the olfactory bulb and the nucleus of the diagonal band were examined for the presence and arrangement of cells labeled with one or both retrograde tracers. In the olfactory bulb both single and double retrogradely labeled mitral cells were found following injections in disparate parts of the olfactory cortex. Furthermore, no consistent pattern was found in the distribution of single- or double-labeled cells in the olfactory bulb; that is, the distribution of cells labeled from one area of the cortex was not consistently different from the distribution of cells labeled from other parts of the cortex. Therefore, it was concluded that individual mitral cells project to widely spaced parts of the olfactory cortex, and that there is no apparent correspondence between the location of a given cell in the olfactory bulb and the distribution of its axon in the cortex. In contrast to this, cells in the nucleus of the horizontal limb of the diagonal band were only rarely double-labeled from nonoverlapping injections into the olfactory cortex or olfactory bulb, although overlapping injections produced a high proportion of double-labeled cells. Cells which were single-labeled from different injection sites were extensively intermixed within the nucleus. Therefore, in this case it was concluded that individual cells projects to relatively restricted areas, although there was again no apparent correspondence between the position of a cell in the nucleus and the terminal field of its axon.