Long-term treatment with an agonist of peroxisome proliferator-activated receptor (PPAR)-gamma is associated with bone fractures in the clinical practice. However, the mechanisms underlying the fractures are not fully understood. This study was aimed to examine the effect of rosiglitazone (an agonist of PPAR-gamma) of different doses on the proliferation, differentiation, and transforming growth factor beta 1 (TGF-beta 1)-induced expression of connective tissue growth factor (CTGF) in primary rat osteoblasts in vitro. Osteoblasts were isolated from newly born SD rats and treated with different doses of rosiglitazone (0-20 mu mol/L). The proliferation and differentiation of osteoblasts were measured by MTT assay and NPP assay, respectively. The expression of CTGF was determined by RT-PCR and Western blotting. The results showed that most isolated osteoblasts displayed strong alkaline phosphatase (ALP) activity and treatment with different doses of rosiglitazone did not affect their proliferation, but significantly inhibited the differentiation of osteoblasts in a dose-dependent manner. Moreover, treatment with different doses of rosiglitazone significantly reduced the TGF-beta 1-induced CTGF mRNA transcription and protein expression in a dose-dependent manner in rat osteoblasts. It was concluded that the activation of PPAR-gamma may inhibit the differentiation of osteoblasts by reducing the TGF-beta 1-induced CTGF expression in vitro.