Formation of bone-like tissue in culture by stromal bone marrow cells (SBMC) derived from young growing rats is dependent on dexamethasone (Dex) (Cell Tissue Res 254:317; 1988) and is significantly enhanced by basic fibroblast growth factor (bFGF) (J Bone Miner Res 8:919; 1993). The aim of this study was to examine the effect of maturation on the osteogenic potential and the response to Dex and bFGF of SBMC by using cultures derived from young growing (6 weeks old) and adult (9 months old) rats. SBMC cultures were grown in the presence of Dex (10⁻⁸ or 10⁻⁷ mol/L) at both P₀ and P₁ and either in the presence or absence of bFGF. The effect of Dex and bFGF on mineralized bone-like tissue (MBT) formation was assessed at P₁. The highest levels of mineralized tissue formation in P₁ subcultures in the absence of bFGF were obtained when cultures derived from young rats (6 weeks old) were treated with Dex 10⁻⁷ and 10⁻⁸ mol/L at P₀ and P₁, respectively, and when cultures derived from adult rats were exposed to Dex 10⁻⁸ mol/L both at P₀ and P₁. Under these optimal Dex concentrations, the amount of MBT formed by adult rat-derived cultures was 15-fold lower than that of young rat-derived ones. The addition of bFGF to P₀ cultures or to P₁ cultures grown under optimal Dex conditions enhanced MBT formation in P₁ cultures derived from both young and adult rats, but this effect was considerably more pronounced in the adult rat-derived cultures. The maximal levels of MBT formation were produced by cultures derived from adult rats treated with bFGF at both P₀ and P₁, whereas in cultures derived from young rats, the addition of bFGF at P₀ was not necessary for maximal MBT production. This stimulating effect of bFGF on MBT formation by adult rat-derived cultures was accompanied by a 2.2-, 1.8-, and 4.3-fold increase in proliferation, alkaline phosphatase activity, and Ca²⁺ deposition rate, respectively. bFGF increased the level of glucocorticoid receptor by approximately 2.3-fold in Dex-treated cultures derived from young animals. These results indicate that maturation is associated with a decrease in the proportion of osteoprogenitor cells in the stromal bone marrow and in their capacity to express the osteogenic phenotype. They further point to the significant role of bFGF in stimulating proliferation and osteogenic expression of stromal bone marrow osteoprogenitors derived from adult rats.