Cyclooxygenase (COX)-1 and -2 catalyze the key reaction in prostaglandin biosynthesis. Whereas COX-1 is found in most tissues, COX-2, with a few exceptions, is not expressed in normal tissues but becomes transiently induced in the course of inflammatory reactions. In many neoplastic epithelia, COX-2 is constitutively overexpressed. Here we show that COX isozymes are spatiotemporally expressed during morphogenesis of dorsal skin epithelium of NMRI mice. COX-1 and COX-2 mRNA and protein were detected in embryonic and postnatal epidermal tissue by RT-PCR, northern blot, and immunoblot analysis indicating that both isoforms may contribute to prostaglandin production. Being barely detectable in interfollicular epidermis and resting hair follicles of adult mice, COX-2 protein appeared in embryonic skin first in epidermal precursor cells and later on in the basal cells and the peridermal layer of the stratified epidermis. In the course of pelage hair follicle morphogenesis, COX-2 remained expressed in the basal interfollicular compartment and, in addition, became apparent in elongated hair germs and hair pegs and later on in the outer root sheath cells of the distal and proximal hair follicles as well as in basal sebaceous gland cells. During the subsequent synchronous phases of hair cycling, COX-2 expression declined in catagen, was barely detectable in telogen, and was reinduced in the basal outer root sheath and basal sebaceous gland cells of anagen hair follicles. COX-1 immunosignals were detected predominantly in the interfollicular spinous and granular layers of the developing, neonatal, and adult epidermis but not in follicular epithelial cells of developing or cycling hair follicles. Dendritic cells in the interfollicular epidermis and distal hair follicles were also COX-1-positive. Transgenic overexpression of COX-2 under the control of a keratin 5 promoter in basal cells of the interfollicular and follicular epidermis induced a precocious entry into the first catagen stage of postnatal hair follicle cycling and a subsequent disturbance of hair follicle phasing. Furthermore, transgenic mice developed an alopecia. Inhibition of transgenic COX-2 activity by feeding the specific COX-2 inhibitor valdecoxib suppressed the development of alopecia, indicating that COX-2-mediated prostaglandin synthesis is involved in hair follicle biology.