Abstract The regulation of synthesis and phosphorylation of osteopontin in relation to avian epiphyseal growth-plate chondrocyte differentiation was studied in situ and in culture. Osteopontin gene expression was evaluated in the tibia growth-plate of 3-week-old chickens by in situ hybridization. The gene was expressed mainly at the lower hypertrophic zone where cartilage matrix is calcified and endochondral bone formation is initiated. Within the hypertrophic region, a poorly labeled area separated the layer of osteopontin-positive hypertrophic chondrocytes from those associated with endochondral bone formation. In culture, proliferative chondrocytes show no alkaline phosphatase activity in contrast to ascorbic acid-treated chondrocytes which display the enzyme activity. Chondrocytes not treated with ascorbic acid, exhibited lower levels of osteopontin mRNA than the treated cells. The phorbol ester TPA—an activator of protein kinase C—and to a lesser extent FGF but not EGF, stimulated osteopontin gene expression. Chondrocytes secreted low levels of phosphorylated osteopontin to the medium. EGF treatment resulted in the appearance of phosphorylated osteopontin in the medium, without affecting the synthesis of other proteins. FGF and TGF β, but not IGF-I or IGF-II, also caused phosphorylation of osteopontin. Ascorbic acid-treated chondrocytes secreted higher levels of phosphorylated osteopontin than the non-treated cells, but addition of FGF or TPA did not stimulate osteopontin phosphorylation any further. Parathyroid hormone caused a dose-dependent attenuation of osteopontin phosphorylation and inhibited the EGF-dependent osteopontin phosphorylation. The results suggest that osteopontin gene expression and phosphorylation in chondrocytes are regulated by separate mechanisms. The response to the various controlling agents varies with the state of differentiation. Both processes—the synthesis and phosphorylation of osteopontin—are under the control of local growth factors which are involved in bone growth and calcification.