After administering 14C-labeled 3-(hydroxymethyl)-8-methoxychromone to rats by gavage, plasma was found to contain unchanged compound and three unconjugated metabolites. These metabolites were identified as 3-carboxy-8-methoxychromone, 8-methoxychromone, and 2-hydroxy-3-methoxyactophenone. The plasma levels of all four labeled compounds were determined from 30 min to 48 hr after drug administration. Only the levels of 3-(hydroxymethyl)-8-methoxychromone and 3-carboxy-8-methoxychromone correlated with the expression of antiallergy activity in the rat, and only these compounds were found to be active in vivo. However, a test system for the inhibition of anaphylactic histamine release in vitro showed activity only for 3-carboxy-8-methoxychromone.