The present study was undertaken to characterize effects of selenium (Se) deficiency on 16 enzymes recovered in either one or more of the subcellular fractions of rat liver (as a basis for future studies on the mechanisms underlying the observed changes). Male rats were fed a Torula-yeast based diet with 0.23 mg Se/kg or the same diet with 0.009 mg Se/kg, from weaning and for 10 weeks. Statistically significant effects of Se deficiency were the following: Se-dependent glutathione peroxidase decreased to 0.14% of the Se-adequate controls, while cytosolic glutathione transferase increased 3-fold in Se deficiency when CDNB was the substrate, but decreased significantly when trans-stilbene oxide (diagnostic for subunit 4) was used as the substrate. Cytosolic DT-diaphorase increased about 7-fold in Se deficiency. Further, DT-diaphorase in the microsomal fraction was also significantly increased in Se deficiency, as were the microsomal and mitochondrial epoxide hydrolases and microsomal glutathione transferase. Furthermore, increased activity of the peroxisomal marker enzyme catalase (P < 0.05) was noted in Se-deficient rats. It is our working hypothesis that changes in enzyme activities in Se deficiency are mainly due to changed levels of endogenously generated metabolites or altered functions of endocrine tissues.