Glutamine uptake into hippocampal slices of the rat was investigated autoradiographically. The characteristics of registered [14C]glutamine uptake such as the incubation with the radiolabelled amino acid at a concentration of 3.5 mumol/l, sodium dependency, the distribution pattern of radioactive material, and the postnatal development of uptake capacity are comparable with those of high-affinity uptake of glutamate. Densitometric evaluation of grain density over hippocampal layers exhibited a marked enhancement of uptake capacity in the neuropil areas during the first postnatal weeks. In the strata oriens and radiatum (CA1) radiolabelling increased from day 2 to 25 by about 390 and 410%, in the strata oriens and lacunosum-moleculare of CA3 by about 350 and 375%, respectively. In contrast, the rise in the accumulation rate in cell body layers was negligible. The temporal and topographical profiles of glutamine uptake in the hippocampal neuropil correlated with those of the activity of phosphate-activated glutaminase and parameters of maturation of the glutamatergic transmission system which have fairly similar time characteristics, suggesting a mutually causative relationship of all these factors.