The olfactory cortices are known as the cortical areas that receive afferent fibers from the olfactory bulb via the lateral olfactory tract (LOT). They include the anterior olfactory nucleus (AOB), the olfactory tubercle (OT) and the piriform cortex (PC). The purpose of the present study is to determine the minimum size of the olfactory cortices necessary for olfactory discriminative task. After ablation of the right olfactory bulb, rats were trained to discriminate between the smell of water and cycloheximide solution, a strong repellent for rodents. After receiving transection injuries on the left LOT, the rats were confirmed to perform olfactory discriminative task. Ibotenic acid (IBO), a neurotoxin, was injected into the olfactory cortices situated between the olfactory bulb and the LOT-transected site, and olfactory discriminative ability of the IBO-injected rats was examined. An anterograde neuronal tracer (BDA) was injected into the left olfactory bulb to distinguish cortical areas with and without bulbar afferents. Area measurements of the intact neuronal profiles of the olfactory cortices of the IBO-injected rats were done in serial brain sections processed for immunohistochemistry for neuronal nuclei (NeuN), a neuronal marker. Neuronal profiles of the AOB, OT and PC with bulbar afferents were measured in the rats with and without olfactory discriminative ability. Minimal olfactory cortical areas of the 3 rats with olfactory discriminative ability ranged between 10%-20% of the unilateral corresponding normal value. We conclude that olfactory discriminative task can be achieved if intact olfactory cortical areas accounted for more than 20% of the unilateral olfactory cortices.