Water extracts from Thesium chinense Tunczaninov (TCTW) and Prunella vulgaris L. (PVW) were tested for aromatase and cyclooxygenase activities. TCTW and PVW were capable of suppressing aromatase in a human placenta microsomal assay. PVW was shown to be more effective than TCTW in the suppression of aromatase activity. TCTW significantly inhibited cyclooxygenase-2 (COX-2) activity at the concentration of 0.25 (p<0.05), 0.5 (p<0.01) and 2.5 mg/ml (p<0.005). PVW also inhibited COX-2 activity in a dose-dependent manner in a concentration range of $0.05{sim}2.5;mg/ml$. The expression of COX-2 was inhibitied by TCTW and PVW in western blot analysis. These results suggest that TCTW and PVW may have breast cancer chemopreventive potentials by inhibiting aromatase and cyclooxygenase activities.