We have studied the expression of TIMP mRNA during mouse embryogenesis and in adult tissues using ribonuclease protection assays and in situ hybridization. Low levels of transcripts were found in many tissues, including embryonic kidney, amnion, lung and maternal deciduum and in these cases expression was not restricted to a phenotypically distinct sub-population of cells. In situ hybridization revealed high levels of TIMP transcripts in the corpus luteum of the adult ovary. Also, we observed significant expression in areas of membrane and endochondral bone formation in the embryo, commencing at about 15.5 d p.c. and increasing until birth. The pattern of TIMP expression in developing bone overlaps significantly with the localization of transforming growth factor beta (TGF beta) implying a role for this factor in the control of TIMP production in vivo.