Glucuronides of drugs are considered to be generally inactive and rapidly eliminated. Therefore, these metabolites are often not taken into account in evaluating drug effects. The present review describes examples of both direct and indirect contributions of glucuronides to net drug effects. Multiple lines of evidence indicate that morphine-6-glucuronide has analgesic activity. This compound has a high affinity to the mu-receptor, is capable of penetrating the blood/brain barrier and is a potent analgesic after administration to patients. Indirect activity of glucuronides may consist of a systemic cycle in which an active parent compound is derived from the glucuronide by enzymatic action. Such systemic cycling has been demonstrated for clofibric acid. In addition, some acyl glucuronides are subject to intramolecular rearrangement and the resulting metabolites are resistant to beta-glucuronidase. Covalent protein binding of glucuronides by different mechanisms may contribute to drug toxicity and immune responses. If glucuronides are accepted as potential modifiers of net drug action it is important to determine what factors modulate disposition of these compounds. Therefore, the later section of this review describes glucuronidation under different pathophysiological conditions. Examples for alterations of the rate and/or extent of glucuronidation by concurrent diseases processes, age and coadministration of other drugs are provided.