The olfactory cortical areas can be largely divided into two parts, the medially located olfactory tubercle (OT) and the laterally located piriform cortex (PC), by the while myelinated bundle (lateral olfactory tract: LOT). We analyses projection patterns from the olfactory bulb to the olfactory cortical areas with different neuroanatomical methods. In the rats that received a small volume (0.1 μl) of injections of a retrograde fluorescent tracer (Fluoro-Gold: FG) into various parts of the olfactory cortex, we first confirmed the diffuse bulbar projection system by the finding that numerous mitral cells in all parts of the bulb were consistently labeled from FG-injected small cortical areas. Secondly, we quantified the total number of mitral cells that were labeled by FG injected into the olfactory cortex. Since the stereology is well established for the whole quantification, we used stereological methods that allow an accurate unbiased sampling. For the total neuronal quantification of FG (+) mitral cells of all sizes, FG (+) mitral cells less than 15 μm were counted in this study. The total numbers of FG (+) mitral cells were 82000, 88000, and 98000 in the rats (n = 3) with large volumes of FG injections into the OT (0.5 μl) and PC (0.5 μl). The total numbers of FG (+) mitral cells were 61000 and 64000 in the rats with small volumes of FG injections into the OT (0.1 μl) or PC (0.1 μl). Surprisingly, mitral cells projecting to small cortical areas accounted for approximately 70% of those neurons projecting to large cortical areas. Thirdly, the combined experiments with bulbar injections of an anterograde neuronal tracer (biotinylated dextran amine: BDA) and incomplete transections of the LOT leaving its medialmost or lateralmost parts showed afferent fibers to both the OT and the PC, indicating that both the lateral and medial LOTs contain traversing fibers projecting to both the OT and PC. From these findings, the bulbar projection system to the olfactory cortex seems very unique in that it almost lacks topographic (point-to-point) organization. This implies that small bulbar areas project to most parts of the olfactory cortex, and, conversely, that small olfactory cortical areas receive inputs from most parts of the bulb.