The c-Fos expression, found in nuclei of neurons, was associated with nociception after many types of noxious stimuli and introduced as a marker of pain. In case of teeth injuries, another detection method of pain sensation in animal models can be inferred from weight loss as an indirect pain measurement of feeding behavior, resulting from pain. The aim of this study was to detect pulpal pain via bodyweight combine with c-Fos expression in the rat model. Ten male Sprague-Dawley rats were nurtured and recorded their pre-operative weights for 1 week. All rats were randomly divided into control and experimental groups. The lower left first mandibular molar was drilled 0.5 mm. in the control group while the pulp was exposed in the experimental group and all rats were parented and recorded their postoperative weights for 1 week. Then, all rats were euthanized and the brain tissues and the teeth were collected. The number of Fos immunoreactive (IR) neurons from the ipsilateral and contralateral brain were detected by immunohistochemistry. The difference of pre- and post-operative increasing weight was analyzed by t-test. Pulpitis was histologically diagnosed. The Fos-IR neurons were significantly higher in the ipsilateral trigeminal nucleus in the experimental group, compared to the control group. In all rats, the post-operative increasing weights increased slower than pre-operative increasing weights and the difference of pre- and post-increasing operative weight of the experimental group was significantly lower than that of the control group. The histologic findings of first molar rats showed normally dental pulp morphology in the control group while the pulp of exposure group exhibited an area of necrosis, enlarged blood vessels and chronic inflammatory cell infiltration. Thus, bodyweight changing can be easily used as an indirect pain measurement of pulpal pain. The methods developed are also useful in pain assessment in animal models.