The vasopressin (VP) and vasoactive intestinal polypeptide (VIP)-expressing neurons in the human suprachiasmatic nucleus (SCN) were morphometrically determined with or without microwave (MW) treatment. Both an enlarged volume and an increased number of neurons were found in the VP and VIP subnucleus of the SCN following MW treatment. The staining of VIP neurons by MW treatment improved much more than that of the VP neurons. Without MW treatment the VP- and VIP-expressing neurons were localized mainly in the central part of the subnuclei, whereas by increasing the sensitivity of the staining by MW treatment more neurons became visible, particularly in the peripheral areas of the subnuclei.