In order to examine the effect of activin A on the process of bone formation, activin A was injected onto the periosteum of parietal bone in newborn rats, and the effect was compared with that of transforming growth factor (TGF)-beta. The daily periosteal injection of activin A increased the thickness of both the periosteal and bone matrix layers in a dose- and time-dependent manner. A maximal effect was obtained with 5.0 micrograms/day activin A. The time course of the effect of activin A on the periosteal thickness was similar to that of TGF-beta 1. However, the effect of TGF-beta 1 was much more pronounced and was mainly on fibroblasts and inflammatory cells. The time course of the effect of activin A on the thickness of bone matrix layer was different from that of TGF-beta 1. The effect of TGF-beta 1 reached maximum (1.8-fold) within 3 days, whereas that of activin A did not develop until day 6 and reached maximum at the end of the 12-day injection period (1.4-fold). Histological examinations revealed that both TGF-beta 1 and activin A increased the number of alkaline phosphatase-positive osteoblastic cells. These results demonstrate that periosteal injection of activin A stimulates bone formation. In addition, although the possibility cannot be ruled out that the dramatic effect of TGF-beta 1 on the periosteal layer might have affected the delivery of TGF-beta 1 to the bone surface, these observations also suggest that the mode of action of activin A may be different from that of TGF-beta 1.(ABSTRACT TRUNCATED AT 250 WORDS)