Previous research has demonstrated that the ventral part of the medial prefrontal cortex, infralimbic area (IL), plays a pivotal role in performing an attention-set shifting (ASS) task, a rodent's version of Wisconsin card sorting test. However, it remains unclear how IL neurons respond to the attentional set-shifting situation. In this study we recorded unit activity of IL neurons during performing an ASS task, in which visual (left or right LED) and auditory stimuli (high or low tone) were simultaneously presented, and only one of two different modality stimuli (LED or tone) was response-relevant (discriminative stimuli). In this task rats were trained to put their noses in either the left or the right hole depending on discriminative stimuli (eg. the right hole to the low tone, the left hole to the high tone). Intracranial electrical stimulation to the medial forebrain bundle was given as a reward 0.5s after the subject had correctly responded to the discriminative stimulus. After the correct response ratio had stably become more than 80%, stimulus-response relationship was shifted to another set (eg. "the right hole for the low tone" to "the right hole for the right LED"). So far, responsiveness of IL neurons recorded were classified into the following 6 types; 1) transient excitation during shifting to new stimulus-response relationship 2) transient inhibition during shifting 3) gradual excitation before presentation of discriminative stimuli and recovering to the baseline level immediately after shifting 4) 5) sustained excitation or inhibition during presentation of discriminative stimuli 6) no significant changes. About 20% of IL neurons recorded exhibited significant changes in firing activity during an early period of attentional set-shifting. None of these neurons showed significant changes in firing activity to each individual discriminative stimulus, while neurons responding to each stimulus were little affected by shifting to a new attentional set. These results suggest that IL neurons involved in set-shifting may be independent of those handling each individual stimulus.