It has been established by organ culture experiments that prostaglandins (PGs) stimulate bone resorption in vitro. Experiments in vivo and with organ cultures suggest that PGs may also stimulate bone formation, and that bone formation in response to a variety of environmental stimuli is PG dependent. We have tested the ability of PGE1, PGE2, and PGF2 alpha to induce bone formation in cultures of rat calvarial cells. PGE1 and PGE2 significantly increased bone nodule formation at concentrations of 10(-8) M and above, to reach 3 times the control levels at 10(-7) M. PGF2 alpha was without effect. The increase in the number of nodules was effected without a significant change in the number of cells in control and test cultures with a logarithmic phase of growth, and there was no increase in the average size of the nodules. This suggests that PGs acted through induction of nodule formation by a population of cells that would not otherwise have produced nodules. Nodules were induced by PGs if the PGs were present in the early stages of the cultures; osteoblastic cells incubated with PGs for 8 days produced very similar numbers of nodules as cultures incubated with PGs throughout the 21-day culture period, although nodules did not become identifiable until 8-10 days of incubation. The addition of PGs late in the culture period had little effect on nodule formation. These experiments identify a role for PGs in bone formation in vitro, which may represent a pathway common to the bone anabolism that is observed in response to many environmental stimuli.