Triamino-substituted 1, 3, 5-triazine derivatives were synthesized and tested for inhibitory activities against the aromatase of human lacental microsomes and the cytochrome P450 side chain cleavage of cholesterol (P450_SCC) of pig adrenal mitochondria. The compounds having imidazolyl and tertiary amino groups as substituents in the 1, 3, 5-triazine ring showed significant aromatase-inhibitory activity. Among them, compounds 17, 23, 26, 27 and 28 were more active than the reference compound, CGS 16949A. The inhibitory activities of these compounds against P450_SCC were much weaker than their aromatase-inhibitory activities. These compounds may be regarded as selective aromatase inhibitors.