Proanthocyanidins, abundant dietary polyphenols made of oligomers and polymers of (epi)catechin units, have strong antioxidant activity and may contribute to reductions in chronic diseases in humans, although their oral bioavailability is incompletely defined. To define pharmacological and toxicological properties of these compounds, there has been considerable interest in the bioavailability and pharmacokinetics of proanthocyanidins present in the diet. Based on LC-MS/MS analysis, we and others have shown that unconjugated proanthocyanidins up to trimers are bioavailable in both plasma and urine after oral feeding of proanthocyanidin-rich extracts. While proanthocyanidin dimer B2 is rapidly absorbed into human plasma as early as 30 minutes after consumption of flavanol-rich cocoa, the highest plasma concentration of the compound by 2 hours is very low (41±4nM). However, after oral administration of [14C] proanthocyanidin B2 to rats, about 60% of the 14C-dose was recovered in the urine within 4 days, suggesting that much of proanthocyanidins administrated orally is degraded by the gut microbiota before absorption. Taken together, the major portion of proanthocyanidins undergo bacterial metabolism in the gut leading to phenolic acids and therefore they may be responsible for health beneficial effects of proanthocyanidins. © 2014 Elsevier Inc. All rights reserved.