The aim of the present study was to examine whether triiodo-L-thyronine (T 3 ) or L-thyroxine (T 4 ) rapidly activated the mitogen-activated protein kinase (MAPK) intracellular signalling cascade in osteoblast-like cells and investigate whether this activation was initiated at the integrin a V b 3 cell surface receptor. Using PCR and western blotting, the expression of integrin a V b 3 mRNA and protein was demonstrated in the human osteoblast-like cell lines MG-63 and SaOS-2. The treatment of MG-63 cells with T 3 (10 nM) or T 4 (100 nM) for 10 min stimulated extracellular signal-regulated kinase activity (ERK, a component of the MAPK pathway) as determined by fluorescent immunocytochemistry and an immunocomplex activity assay (T 3 by 10 . 7-fold, P!0 . 01 and T 4 by 10 . 4-fold, P!0 . 01 compared with control). T 3 (10 nM) and T 4 (100 nM) also significantly stimulated thymidine incorporation into MG-63 cells by 2 . 3G0 . 7-fold (P!0 . 01) and 2 . 1G0 . 1-fold (P!0 . 05) respectively. To establish whether transient ERK activation via the integrin a V b 3 cell surface receptor mediated these effects, MG-63 cells were pretreated for 30 min with the specific MAPK kinase inhibitor, U0126 (1 mM), or an antiintegrin a V b 3 -blocking antibody. Both pretreatments significantly inhibited T 3 -and T 4 -stimulated ERK activation and abolished T 3 -stimulated thymidine incorporation (P!0 . 01).T 4 -stimulated incorporation was significantly inhibited from 2 . 1-to 1 . 3-fold above control (P!0 . 05). Thus, our results suggest that T 3 and T 4 rapidly stimulate ERK activation in MG-63 cells via integrin a V b 3 and that one functional effect of this ERK activation is increased DNA synthesis.