Five tetrahydroxyxanthones, 3,4,6,7-tetrahydroxyxanthone [1], 1,3,5,6-tetrahydroxyxanthone [2], 3,4,5,6-tetrahydroxyxanthone [3], 1,3,6,7-tetrahydroxyxanthone [4], and 2,3,6,7-tetrahydroxyxanthone [5] isolated from Tripterospermum lanceolatum inhibited angiotensin-I-converting-enzyme activity in a dose-dependent manner. The mode of inhibition of the tetrahydroxyxanthones (THXs) was found to be competitive inhibition. When the tetrahydroxy groups of THXs were blocked with acetyl groups, the angiotensin-I-converting-enzyme inhibitory activity was abolished, suggesting that the tetrahydroxy groups are indispensible for the inhibitory activity.