Guanine nucleotide exchange factor (GEF) activity in ribosomal high salt wash and cytosolic fractions from suckling (4-10-day-old) and adult (60-day-old) rats was assayed by two different methods, by measuring: (i) its ability to promote binding of [3H]Met-tRNAi to eukaryotic initiation factor-2 (eIF-2) preparations that are partially or wholly in the form of eIF-2-GDP complexes (at Mg2+ concentrations near the optimum for protein synthesis), and (ii) under similar conditions, its ability to catalyze the displacement of [3H]GDP, previously bound to eIF-2, by unlabelled GDP. A purified eIF-2 (GEF-free) from brain was used as the source of eIF-2 activity. GEF activity in ribosomal fractions is higher in the brain of suckling than adults rats, and a direct correlation therefore exists between ribosomal GEF activity and the previously observed age-related decrease in eIF-2 activity in ribosomal high salt wash protein fractions. On the other hand GEF activity in the postmicrosomal supernatant is lower in the brain of suckling than adult rats. These findings further support the hypothesis that the progressive decrease in protein synthesis during brain development is controlled through regulation of the initiation step, by modulation of eIF-2/GEF activities.