It is well documented that prostaglandin E2 (PGE2) has the ability to stimulate bone formation, improve bone structure, and increase bone mass in intact or osteopenic rat models. However, the effects of PGE2 on the mechanical properties of bone have not been investigated previously. The purpose of our study was to determine the effects of PGE2 on the mechanical strength of bones in rapidly growing, adult, and ovariectomized rat models. In study I, PGE2 at 3 mg/kg per day, or vehicle, was given by daily subcutaneous injections for 30 days to rapidly growing (3-month-old) intact male rats. Compared with controls, PGE2 significantly increased initial maximal load and stiffness of cancellous bone at the distal femoral metaphysis (DFM) as determined by an indentation test. As determined by a compression test, rats treated with PGE2 showed a significant increase in maximal load, and a nonsignificant increase in stiffness in the fifth lumbar vertebral body (L5) when compared with controls. In study II, PGE2 at 3 mg/kg per day, or vehicle, was given by daily subcutaneous injection for 30 days to mature (10-month-old) intact male rats. PGE2 treatment significantly increased initial maximal load and stiffness of the DFM and L5. PGE2 induced a significant increase in maximal load, but not stiffness, in the femoral neck (FN), as determined by a cantilever compression test. There was an increase in maximal load in a three-point bending test at the femoral shaft (FS) although the increase did not achieve statistical significance. No change in stiffness in the FS was found after PGE2 treatment. In study III, 3-month-old female rats were sham-operated or ovariectomized (ovx) for 30 days. Thereafter, PGE, at 1 or 3 mg/kg, or vehicle, were given by daily subcutaneous injection to these rats for 30 days. After 30 and 60 days, ovx induced a significant decrease in initial maximal load and stiffness of cancellous bone at the DFM as compared with sham controls. In ovx rats with established osteopenia, PGE2 at 1 mg/kg per day nonsignificantly increased the initial maximal load and stiffness, whereas, at 3 mg/kg per day, PGE2 completely restored the initial maximal load and stiffness of DFM to sham control levels. Similarly, maximal load and stiffness of L5 decreased significantly in ovx rats compared with sham controls at 30 days postsurgery. PGE2 at 1 mg/kg per day partially restored the maximal load, whereas, at 3 mg/kg per day, it completely restored the maximal load and stiffness of L5 in the established osteopenia, ovx rats. At the FS, PGE2 at 3 mg/kg per day nonsignificantly increased maximal load (+11%) and significantly increased stiffness (+25%) compared with ovx controls. Neither ovx nor PGE2 treatment caused a significant change in the maximal load and stiffness of the FN in this study. These results reveal that PGE2 significantly increased the mechanical strength at various skeletal sites in rapidly growing and mature male rats, although the increase in femoral shafts was not statistically different. Furthermore, PGE2 completely restored mechanical strength to the cancellous bone in ovx rats with established osteopenia.