The distribution of c-fos proto-oncogene expression has been studied in the periaqueductal grey matter (PAG) of non-intentional-stimulated rats by immunohistochemistry. A mean number of 53 +/- 9 Fos-like immunoreactive cells per hemiPAG, distributed into three groups, dorsolateral, lateral and ventrolateral, was found. The dorsolateral and the lateral groups appeared along the entire rostrocaudal PAG, whereas the ventrolateral group appeared only in the caudal half. These results reveal new data on the number and extent of the longitudinal columns within the hypothetical organization of the PAG. They also indicate a continuous activation of a significant population of neurones in the PAG, in agreement with the suggested role of the PAG in the modulation of internal information.