To study possible functional involvement of gamma-glutamyl transpeptidase (GGT) in glutamate transmitter metabolism we lesioned putative glutamatergic structures of the rat hippocampal formation by intracerebroventricular (i.c.v.) injection of kainic acid (KA) or by surgical CA3 axotomy. Unilateral injection of KA into the left lateral cerebral ventricle of 30-day-old rats resulted in decreased GGT activity in hippocampal areas CA3, Ca1 ipsilaterally, and in the contralateral area CA1, four hours after the induction of the chemical lesion. Four days after the injection, the enzyme activity was decreased in all hippocampal areas with the exception of the contralateral dentate gyrus. Four days after bilateral i.c.v. injection of KA, lower GGT levels were found than was seen after bilateral surgical lesion of the CA3 pyramidal cell axons (Schaffer's collaterals). The surgical lesion was followed by a decrease of GGT only in the stratum pyramidale and stratum radiatum of area CA1. In contrast to the effects in 30-day-old rats, unilateral i.c.v. injection of KA on postnatal day 12 did not alter the GGT activity in any studied hippocampal area presumably because of incomplete maturation of structures required for KA vulnerability.