Hyaluronan, a major extracellular matrix component in the epidermis, has been shown to control keratinocyte proliferation and differentiation in vitro. We assayed hyaluronan and hyaluronan synthases (has1-3) in mouse epidermis during fetal development, postnatal life, and trauma reaction in vivo. Hyaluronan increased in the epidermis when keratinocytes started to stratify on day E15, remained high until birth, and then rapidly declined, with corresponding changes in the mRNA levels of has2 and has3. The hyaluronan in adult mouse epidermis mainly resided around the orifices of the hair follicles, and the overall concentration was about one order of magnitude lower than in adult human epidermis. In adult mice, epidermal trauma caused by tape stripping rapidly increased hyaluronan, leading to a 6-fold increase in epidermal hyaluronan on day 3 following trauma. The hyaluronan response was associated with a strong induction of has2 and has3 mRNA, slightly higher CD44 expression, and considerable epidermal hyperplasia. The data show that the pre- and postnatal fluctuations in epidermal hyaluronan content correlate with the expression levels of has2 and has3. Stimulated hyaluronan synthesis through upregulated has expression is an inherent feature of the keratinocyte activation triggered by tissue trauma, and presumably important for a proper healing response.