To evaluate the transcriptional response of skin to tissue expansion in the dog, the expression of procollagen alpha 1(I) mRNA and procollagen alpha 1(III) mRNA were analyzed by in situ hybridization. This expression was evaluated in distinct skin areas (subepidermal zone, dermis, capsular zone) after 4-85 days of expansion. Within the first 4 days of expansion expression of procollagen alpha 1(I) and alpha 1(III) mRNA was not affected in the subepidermal and dermal zone. Only a slightly elevated level of type III procollagen mRNA was demonstrated in tissue surrounding the implanted silicone expander. After 7 days of expansion an enhanced level of procollagen alpha 1(III) mRNA was observed in the dermis and capsular zone. A slightly enhanced type I collagen mRNA level occurred in all zones of the dermis that was even stronger in the capsular zone after 9 days of expansion. Concurrently, the number of transcriptionally active cells was significantly higher. Return to the basal level of procollagen alpha 1(I) mRNA was attained after 40 days. At this time a significant expression of procollagenase mRNA was observed. Procollagen III mRNA expression reached its basal level on day 85.