Glutathione S-transferase (Gst) enzymes are instrumental in protecting cellular macromolecules against electrophiles and products of oxidative stress. Of interest primarily to pharmacologists and toxicologists is the ability of these enzymes to metabolize cancer chemotherapeutic drugs, insecticides, herbicides, and carcinogens. Thus, constitutive expression of Gsts might determine a tissue's ability to handle certain forms of chemical stress. In the present study, the constitutive mRNA expression of 19 different Gst enzymes was investigated in 14 different tissues in mice. The information obtained from the present study could be distilled into a few generalized principles: in all tissues examined, multiple isoforms of Gst were constitutively expressed; several isoforms, such as Gstk1, Gstm1, Gstm4, Gstm6, and Gstt1, were expressed in most of the tissues studied; at least five Gst isoforms were highly expressed in the gonads, about three in heart, and at least one in brain (Gstm5). Gender differences in the expression of various Gst isoforms were pronounced. With a few exceptions, most of the Gst isoforms expressed in kidney showed higher expression in females than males; the same trend was observed for heart and gonads. At least eight Gst isoforms showed very high expression in stomach. This was a unique finding in the current study because drug-metabolizing enzymes that are highly expressed in the gastrointestinal (GI) tract tend to have the highest expression in small intestine with low or no expression in the stomach. In summary, most Gst isoforms are most highly expressed in the GI tract and liver, which strongly suggests an important role of many Gst isoforms in detoxification of ingested xenobiotics.