Hepatic glutathione S-transferase (GST) activities towards 1-chloro-3,4-dinitrobenzene (DNCB), 3,4-dichloronitrobenzene (DCNB), sulfobromophthalein (BSP), p-nitrobenzyl chloride (NBC), ethacrynic acid (EA), trans-4-phenyl-3-buten-2-one (TPBO) and 1,2-epoxy-3-(p-nitrophenoxy)propane (ENPP) were determined in mice, rats, rabbits and guinea-pigs during ageing and after pretreatment with enzyme inducers. Variations were observed in the developmental patterns and in the phenobarbital-, benzo(a)pyrene-, pregnenolone-16 alpha-carbonitrile-, butylated hydroxyanisole-, trans-stilbene oxide-inducibility of hepatic GST activities in the same species towards different substrates. For example, in rats GST activities for EA, DCNB and TPBO increased respectively, 2.3-, 4.8- and 25-fold during age-development, and after treatment with TSO 1.2-, 3.6- and 1.3-fold. Species differences were found in the maturation and in the inducibility of GST activities. For instance, GST activity toward EA at birth is mature in guinea pigs but not in the other species; phenobarbital treatment increased GST activities in mice and rats but not in rabbits and guinea-pigs; treatment with trans-stilbene oxide enhanced GST activity for TPBO 4.5-fold in mice but not at all in rats. It is concluded that hepatic glutathione conjugation exhibits functional heterogeneity which may be due to species dependent variations in the responsiveness of GST isoenzymes to endogenous and exogenous influences.