Uridine 5'-diphosphate-glucuronosyltransferases (UGTs) are the biological catalysts of glucuronidation, a major pathway of conjugative metabolism of drugs and xenobiotics. In addition to the liver and kidney, UGTs are highly expressed in the gastrointestinal tract, where they have the potential to influence the pharmacokinetics and biological effects of ingested drugs and xenobiotics. This paper reviews the current evidence for the contributions of intestinal UGTs to presystemic 'first-pass' metabolism and drug bioavailability, the extent of enterohepatic cycling and the clearance of drugs from plasma, as well as their influence on biological responses to drugs, including drug toxicity. The prediction of the effects of intestinal glucuronidation on these processes depends on knowledge of the types and amounts of UGTs expressed in the small intestine and their specific glucuronidating activities. Whereas the types of UGTs expressed in human gastrointestinal tract are well characterized, further research is needed to understand the absolute amounts of UGTs in the small intestine and the causes of observed high-interindividual variability in the intestinal expression of UGTs.