Thirty-one flavonoids were tested for their effects on low Km phosphodiesterase with cyclic AMP as the substrate. Quercetin, luteolin, scutellarein, phloretin and genistein showed inhibitory potencies comparable to or greater than 3-isobutyl-2-methylxanthine (EC50 30-50 microM). Only four compounds namely, catechin, epicatechin, taxifolin and fustin stimulated the enzyme activity (stimulatory EC50 130-240 microM). The most potent phosphodiesterase (PDE) inhibitors were aglycones that had a C2.3 double bond, a keto group at C4 and hydroxyls at C3' and/or C4'. However, when the C-ring is opened then the requirement for the C2.3 double bond is eliminated. The same series of flavonoids were also tested for their lipolytic activity. The structural features required for effective synergistic lipolysis (with epinephrine) were generally similar to that required for potent PDE inhibition except that, for lipolytic activity, an intact C-ring was necessary. Fisetin and quercetin having the above-mentioned structure showed a dose- and time-dependent increase in lipolysis which was synergistic with epinephrine. Only butein and hesperetin showed inhibition of epinephrine-induced lipolysis, and their effect was dose-dependent. A time-course study indicated that hesperetin was able to delay the lipolytic action of epinephrine. It is most likely that the lipolytic effects of these compounds were not a result of PDE inhibition, as the orders of potency for the two activities had poor correlation. Apparently, the effective lipolytic flavonoids were also potent PDE inhibitors but not all the PDE inhibitors were able to induce lipolysis.