Objective To investigate the insulin-like growth factor-Ⅰ (IGF-Ⅰ) gene and polypeptide expression in cultured rat osteoblast (ROB) and the role of IGF-Ⅰ in mediating the cell-to-cell communication by mimicking the pharmacokinetics of parathyroid hormone (PTH).Methods The ROB was cultured with three kinds of treatment: (1) Control (Ctr), the cells were cultured without PTH during the first 6 hours and the subsequent 42 hours in a 48-hour cycle; (2) Intermittent exposure to PTH (Itm), the cells were cultured with PTH during the first 6 hours, but without PTH in the subsequent 42 hours; and (3) Continuous exposure to PTH (Ctu), the cells were cultured with PTH during the first 6 hours and the subsequent 42 hours.Results The bone-forming activities of ROB were increased in Itm and inhibited in Ctu. The IGF-Ⅰ mRNA content in Itm cells was elevated only during the first 6 hours and that in Ctu cells was elevated at any time during an incubation cycle. The free IGF-Ⅰ concentration in the medium of Itm cells was generally higher and that of the Ctu cells was generally lower compared with those of the Ctr cells. The IGF-Ⅰ antibody significantly reduced the alkaline phosphatase activity within the cells of Ctr and Itm.Conclusions PTH rapidly and constantly stimulates the IGF-Ⅰ gene transcription of osteoblast. There was an obvious discrepancy between the IGF-Ⅰ mRNA content within the osteoblast and the free IGF-Ⅰ level around the osteoblast in either mode of PTH action. The IGF-Ⅰ might be important for osteoblast-osteoblast communication and bone-forming activity, not only in intermittent PTH administration, but also in the physiological functioning of osteoblasts.