1. The degradation of tolmetin glucuronide (TG) in biological fluids and tissue homogenates appears to follow first-order kinetics and is quite rapid in plasma. TG degradation was minimized upon the addition of phenylmethylsulphonyl fluoride (PMSF) and 1,4-saccharolactone, suggesting that the majority of the degradation may be enzymatic, rather than chemical hydrolysis. 2. Irreversible binding via TG was detected in all tissue preparations examined. Upon addition of an inhibitor of esterases (PMSF) to human serum albumin (HSA) and plasma, binding was extensive (2.5%) and the extent of binding was both time- and pH-dependent. Similar extents of binding were obtained with most tissue homogenates, except for spleen and intestine which exhibited much lower binding. 3. Incubation of TG with microsomal protein from sheep and rat yielded no significant differences. Incubations of tolmetin (T) and TG with microsomes, as well as tissue homogenates, indicates that irreversible binding occurs only in the presence of TG. 4. Irreversible binding occurred in all of the blood constituents, the highest extent with haemolyzed erythrocytes. The extent of binding was 15 times higher in disrupted versus intact red blood cells, suggesting a correlation between the extent of binding and the overall exposure of TG to the macromolecules to which it may bind irreversibly.