There is evidence that glutathione conjugates are the major metabolites formed following systemic uptake of carcinogenic contaminants from the intestine. The effect of commercial diet versus a semipurified diet on the distribution of glutathione S-transferase (GST) activity was examined in proximal, medial, and distal sections of catfish intestine. The bulk of GST activity with 1-chloro-2,4-dinitrobenzene, ethacrynic acid, and 3 H-benzo[a]pyrene-4,5-oxide, and the percent cytosolic protein cross-reacting with anticatfish GST-p were in the more proximal segments and dropped off distally in the two diet groups. However, the total GST-p cross-reacting protein in the proximal section was significantly higher in fish fed a chow diet. Western blot analysis revealed p-class GST to be expressed principally in the proximal intestine. Cytosol samples cross-reacted with antibodies to human GST-โฃ, -l, and -p, but not -h, classes. Alpha-like GST isoforms of MW 26,200 and 24,600, absent in sections from fish fed a purified diet, were differentially expressed only in the distal section of chow-fed fish. These results indicate that diet significantly elicits regional differences in GST protein levels, that components of the commercial chow affect GST protein expression in the distal intestine, and that maintenance diet should be taken into consideration during dietary exposure studies. @