Mouse colon adenocarcinoma Co38 is widely used as a screening model for human colon tumors. To understand better the influence of tumor size on the main drug-metabolizing enzyme systems, we tested 15 mouse Co38 tumors at different sizes. The average weight was 917±444 mg (range, 300–1,400 mg). Cytochromes P-450 (1A1/1A2, 2B1/B2, 2C8–10, 2E1, 3A4), epoxide hydrolase (EH), and glutathione-S-transferases (GST-α,-μ, and-π) were assayed by immunoblotting. The activities of the following enzymes or cofactors were determined by spectrophotometric or fluorometric assays: 1-chloro-2,4-dinitrobenzene-GST (CDNB-GST), selenium-independent glutathione peroxidase (GPX), 3,4-dichloronitrobenzene-GST (DCNB-GST), ethacrynic acid-GST (EA-GST), total glutathione (GSH), uridine diphosphate-glucuronosyltransferase (UDP-GT), β-glucuronidase (βG), sulfotransferase (ST), and sulfatase (S). Our results showed the absence of all probed P-450s and EH in Co38 tumors. No relationship was found between the Co38 tumor weights and GPX, GST-α, and EA-GST (regression analysis). However, a significant correlation was found between the tumor weights and all other enzymes investigated. For certain enzymes or cofactors, a linear decrease (P<0.05) was observed as a function of tumor weight (CDNB-GST, DCNB-GST, GST-μ, GST-π, GSH, and βG). Other enzymatic activities (UDP-GT, S, and ST) were found to decrease in medium-size tumors and to increase in large tumors (P<0.05; quadratic correlation). These data demonstrate that the expression of many drug-metabolizing enzyme systems is altered during tumor growth and suggest that tumoral response to chemotherapy could be altered as a function of tumor size.