Prostaglandin E2 (PGE2) has been shown to possess anabolic properties when administered systemically. All the experiments performed so far examined long bones from animals of varying age and bone status. In this study we compared the changes in bone mass of long bones (femur, tibia and humerus) to those in calvariae after a 3-week daily administration of 6mg/kg PGE2 into 3-week-old rats. This regimen inhibited body weight gain (by 14· 1%) as well as longitudinal growth of long bones (by 2· 2–3· 5%) but increased their mass. Ash weight (measuring both cancellous and compact bone) increased by 10· 1–14· 1% but tibial cancellous bone area was elevated by 54%. Radial growth was slightly reduced due to transient inhibition of mineral apposition rate at the periosteal envelope but the expansion of the marrow cavity was inhibited to a greater extent, resulting in an 8· 1% increase in the relative compact bone area. The increased bone mass was associated with greater mechanical strength of the femoral neck (24· 2% increase in fracture load and 19% in stiffness). In contrast, PGE2 administration did not affect calvarial thickness or mineral apposition rate but increased its density, ie reduced the area of marrow spaces due to stimulation of endocortical bone formation at this site. The pattern of bone mass changes documented in this study closely correlates with that of the induced expression of early-response genes following a single dose of PGE2 as we recently reported. These data, therefore, support the hypothesis that in vivo administration of an anabolic dose of PGE2 increases bone formation and augments bone mass largely by stimulating the recruitment of new osteoblasts via induction of the proliferation and/or differentiation of bone marrow osteogenic precursors.