Paraquat (1 mM), when added to isolated haemoglobin-free perfused rat liver, leads to an increase of intracellular mixed disulphides from 1.3 mumole GSH equivalents per g wet weight in the controls to 2.5 mumole/g. This raises the proportion of mixed disulphides to total glutathione equivalents from about 0.2 at the beginning of the perfusion to about 0.4. The mixed disulphides are predominantly protein-bound, with low molecular weight compounds being quantitatively negligible. The content of intracellular glutathione disulphide (GSSG) is increased from 17 nmole/g in the controls to 38 nmole/g in the presence of paraquat. In addition, there is an increased rate of release of GSSG into the extracellular (biliary) space, reported previously. It is suggested that, in a reaction catalysed by thioltransferase(s), the rise in GSSG is correlated with the rise in mixed disulphides (reaction 1). Occupancy of potential cellular mixed disulphide sites is about 1/2 in the controls, and rises to about 2/3 in the presence of paraquat. THe ratio of cellular contents, NADPH/NADP+, is decreased from 5.1 in the controls to 2.3 in the presence of paraquat, while the sum of NADPH plus NADP+ remains unaltered. The perturbation in the glutathione status may be related to metabolic effects such as the stimulation of the pentose-phosphate pathway activity, and possibly also to the expression of toxic effects.