Three case presentations illustrate clinically relevant pharmacokinetic properties of morphine. Morphine given orally is rapidly absorbed and transformed to morphine-glucuronides (M3G, M6G) by the liver. The polar metabolites are excreted by the kidney. In chronic or acute renal insufficiency biologically active metabolites of morphine accumulate in the serum and may be responsible for severe toxic side effects. Hepatic insufficiency decreases the first-pass effect and therefore increases the bioavailability of oral morphine. This has to be considered when changing from parenteral to oral administration of morphine.