We examined which substances stimulate hyaluronic acid synthesis in rabbit pericardium and pericardial cells. (1) The addition of 10% (v/v) pericardial fluid to the incubation medium of the pericardium enhanced hyaluronic acid synthesis to about 2.5-times over the control, suggesting that there were some stimulating factors of the hyaluronic acid synthesis in the pericardial fluid. (2) The addition of 100 ng/ml insulin-like growth factor-I (IGF-I) to the incubation medium caused an approx. 2-fold increase of hyaluronic acid synthesis over the control in the pericardium. (3) IGF-I stimulated the hyaluronic acid synthesis in pericardial cells in a dose-dependent manner, but did not significantly stimulate the sulfated glycosaminoglycan synthesis. (4) The increase in the hyaluronic acid synthesis induced by IGF-I (100 ng/ml) or pericardial fluid was blocked by the copresence of monoclonal anti-IGF-I antibody. (5) IGF-I caused possible induction of the hyaluronic acid synthase activity and general protein and RNA synthesis before the initiation of DNA synthesis in pericardial cells. These results suggest a new biological action of IGF-I that stimulates hyaluronic acid synthesis in pericardium or the pericardial cavity of heart.