The corticopontine fibres were severed in the crus cerebri in rats and mice by a stereotaxically operated retractable wire-knife. The pontine nuclei were microscopically dissected from fresh slices of rats and synaptosome-containing homogenates were prepared. The high affinity uptake of radiolabelled L-glutamate (L-Glu) and D-aspartate (D-Asp) was heavily reduced five days after the lesions. The uptake was further reduced after bilateral (-75% for D-Asp and -65% for L-Glu) than after unilateral lesions (-55% for D-Asp and -45 to 50% for L-Glu on the lesioned side.) The molar ratio of the uptakes of D-Asp and L-Glu was consistently lower in pons after transection of the cortical afferents than normally (-28% after bilateral lesions). gamma-Aminobutyrate uptake and glutamic acid decarboxylase were not changed. Choline acetyltransferase was increased (+53%) after unilateral lesions, but not altered after bilateral lesions. Autoradiograms of slices from mice, incubated with tritium-labelled amino acids and fixed in glutaraldehyde, showed high affinity uptake sites for D-Asp to be enriched in the pontine nuclei, compared to neighbouring structures. After partial lesion of the crus cerebri the uptake was reduced in the area with degenerated corticopontine afferents. gamma-Aminobutyrate uptake sites were relatively less concentrated in the pontine nuclei than D-Asp uptake sites. The results indicate, along with the previous demonstration of Ca-dependent K-induced release of D-[3H]aspartate from the corticopontine terminals, that glutamate and/or aspartate may be transmitters in this pathway. The results also suggest that acidic amino acid uptake sites may differ in their relative transport rates for aspartate and glutamate.