Alkyl gallate, which is known as an antioxidant, intensively inhibited delta 5 and delta 6 desaturation in both rat liver microsomes and an arachidonic acid-producing fungus Mortierella alpina 1S-4. The rat liver microsomal delta 5 and delta 6 desaturases were inhibited by gallic acid esterified with alcohols with various numbers of carbons, suggesting that the necessary structure in an esterified alcohol for the inhibition is not so strict. Among the three hydroxy groups in gallic acid, the m-hydroxy group was shown to be the necessary structure. Kinetic analyses revealed that propyl gallate is a noncompetitive inhibitor of delta 5 desaturase (Ki = 2.6.10(-5)M) and delta 6 desaturase (Ki = 1.7.10(-4) M). These data indicate that alkyl gallate is a new type of desaturase inhibitor and different from known natural inhibitors, i.e., sesamin and curcumin.