Glutamate is the main excitatory transmitter in the cerebral cortex. The physiologically high spatial and temporal resolution in glutamatergic transmission requires effective transmitter removal. Thus, a close topochemical relation to the glutamatergic synapse is a prerequisite for an enzyme involved in glutamate transmitter degradation. Here we report that immunoreactivity against glutamine synthetase (GS), one of the glutamate metabolizing enzymes, is localized in the fine astrocytic processes associated with identified glutamatergic synapses in the rat hippocampus. We suggest that glutamate transmitter is rapidly taken up by these fine perisynaptic astrocytic processes and degraded by GS.