The hypoxia-inducible factor-1 (HIF-1) has been known to be correlated to the adaptation and proliferation of tumor cells; therefore HIF-1 has become an important target in the development of anticancer drugs. A phytochemical study of the CHCl3-soluble fraction of Salvia miltiorrhiza, which strongly inhibited hypoxia-induced reporter gene expression, led to the isolation of 12 abietane-type diterpenes. Of these compounds, sibiriquinone A (1), sibiriquinone B (2), cryptotanshinone (3), and dihydrotanshinone I (4) potently inhibited hypoxia-induced luciferase expression with IC50 values of 0.34, 3.36, 1.58, and 2.05 microM on AGS cells, a human gastric cancer cell line, and 0.28, 3.18, 1.36, and 2.29 microM on Hep3B cells, a human hepatocarcinoma cell line, respectively. Consistently, 1 and 4 dose-dependently suppressed the HIF-1alpha accumulation and 1 inhibited mRNA expression of vascular endothelial growth factor (VEGF) under hypoxia. These results suggest that the anticancer activity of tanshinones is likely at least in part associated with their inhibition of HIF-1 accumulation.