Prostaglandin-H synthase (PGHS)-1 and -2 expression in mouse skin and in keratinocytes in culture was determined using immunohistochemistry and Western blot analysis. In normal skin PGHS-1 immunoreactivity was found in individual keratinocytes present in the interfollicular epidermis and the upper part of the hair follicle. PGHS-2 immunostaining was detected in very few individual basal cells of the interfollicular epidermis and of the hair follicle. Upon induction by TPA of an inflammatory epidermal hyperplasia (regenerative hyperplasia) the number of PGHS-2-expressing keratinocytes scattered throughout the basal but not the suprabasal compartment of the interfollicular epidermis was found to be increased while PGHS-1 expression remained unchanged. PGHS-2 immunoreactivity in paraffin sections from TPA-treated skin showed a nuclear in some and a perinuclear and cytoplasmic localization in other keratinocytes. This different distribution may correlate with the proliferative state, since immunofluorescence analysis of mouse keratinocytes in culture demonstrated a predominant perinuclear and cytoplasmic PGHS-2 localization in cycling keratinocytes but a prevalent staining of the nucleus and the nuclear membrane in noncycling cells. Stimulation of proliferation of murine primary keratinocytes by serum resulted in an increased PGHS-2 expression, whereas induction of terminal differentiation by Ca²⁺caused a down-regulation of PGHS-2 protein. Only minor changes in PGHS-1 expression were seen. Our data suggest that expression of PGHS-2 in mouse skin epidermis is related to epithelial regeneration.