The postnatal development of glutathione S-transferase activities was investigated in post-microsomal supernatants of the livers of male rats. A period of up to the age of 200 days was covered, using styrene 7,8-oxide as the electrophilic substrate for the transferase. Activity in animals on the day of delivery was small but significantly above the level of spontaneous conjugation. During 5 weeks postpartum the specific activity increased regularly to about sixfold when expressed as per mg of cytosolic protein, thereafter the activity slightly decreased. During the 5th postnatal week a sudden decrease in activity was observed, but it returned to the original value within about 5 days. The effect of weaning on this phenomenon was studied by comparing weaned with non-weaned rats in an investigation of glutathione S-transferase maturation in the 4th and 5th week postpartum, using styrene 7,8-oxide and 1-chloro-2,4-dinitrobenzene as the electrophilic substrates. Weaned rats exhibited the decrease in activity only when styrene oxide was the substrate. It is concluded that glutathione S-transferase levels in rat liver are low at birth, and develop gradually during the first 5 postnatal weeks. Transferase activities towards styrene oxide and chlorodinitrobenzene appears to mature independently and are partly associated with weaning.