tRNA was extracted from brains of 3-, 8-, and 18-day-old rats that were injected intracerebrally, 45 min before death, with [3H]methyl methionine or [8-3H]guanosine, and intraperitoneally, 3 h before death, with L-methionine-dl-sulfoximine (MSO), a methylation-activating convulsant agent. Although there was no effect of age or of MSO on the per gram yield of tRNA, its specific radioactivity (dpm/A260) was highest at 3 days in both the control and the MSO groups. Age- and MSO-related changes in tRNALys content of the brain tRNA pool were investigated by means of benzoylated DEAE-cellulose (BDC) and reverse-phase chromatography (RPC). BDC chromatography revealed tRNALys species in the brains of the MSO-treated animals that were absent in control brains. Of particular interest was the finding that differences in RPC-5 chromatographic mobility between control and MSO-tRNALys species were abolished by conversion of lysyl-tRNA, suggesting that the MSO-elicited change(s) in tRNALys structure involved the binding site(s) for lysine. Two additional findings were made: (a) lysine acceptance by the [3H]methyl-labeled tRNALys purified from brains of the MSO-treated animals was higher than that of controls at 18 days; and (b) omission of the BDC chromatographic step accentuated the differences in mobility on RPC-5 columns between tRNALys species of control and MSO-treated animals. Lastly, we found that some tRNALys species of control and MSO-treated brains contained significantly different proportions of N2-methyl guanine and 1-methyl adenine, relative to controls. These MSO-elicited changes in the methyl base content of tRNALys of immature rat brain are the first evidence of an alteration of brain tRNA structure by a centrally acting excitatory agent.