Changes in Fos expression of magnocellular neurons in the supraoptic nucleus (SON) and the paraventricular nucleus (PVN) of the rat hypothalamus were investigated using immunohistochemistry during pregnancy, parturition and lactation. Quantitative morphometrical analysis revealed that Fos-positive cells in the hypothalamic magnocellular neurons were rarely seen in days 10 and 20 pregnant rats, however, significantly numerous Fos-positive cells were found in parturient rats. The number of Fos-positive cells was drastically decreased within a few days after parturition. Moreover, it was found using dual immunohistochemistry that the percentage of Fos-positive cells in vasopressin (AVP) magnocellular neurons of the SON was higher than that of the PVN in parturient rats, although oxytocin (OXT) magnocellular neurons showed the same percentage of Fos-positive cells between the SON and PVN. These results demonstrate that the hypothalamic magnocellular neurons express Fos during a limited period after parturition, and Fos expression in AVP magnocellular neurons is heterogeneous between the SON and PVN.