The expression of growth-associated protein-43 has been related to axonal elongation and synaptic sprouting. Using the Northern blot analysis, we investigated the developmental changes of growth-associated protein-43 mRNA in the thalamus of macaque monkeys. The amount of growth-associated protein-43 mRNA was high at embryonic day 125, and decreased at postnatal day 1. It increased again at postnatal day 8, reached its peak value at postnatal days 50-70, and then decreased gradually until postnatal year 1. We previously reported that the amount of growth-associated protein-43 mRNA in the cerebral cortex decreased roughly exponentially during perinatal and postnatal periods and that it approached the asymptote by postnatal day 70 [Oishi T, Higo N, Umino Y, Matsuda K, Hayashi M (1998) Development of GAP-43 mRNA in the macaque cerebral cortex. Dev Brain Res 109:87-97]. The present findings may indicate that extensive synaptic growth of thalamic neurons continues even after that of cortical neurons has finished. We then performed in situ hybridization to investigate whether the expression level of growth-associated protein-43 mRNA was different among various thalamic nuclei. In the infant thalamus (postnatal days 70-90), moderate to intense expression of growth-associated protein-43 mRNA was detected in all thalamic nuclei. Quantitative analysis in the infant thalamus indicated that the expression levels were different between the nuclear groups that are defined by the origin of their afferents. The expression in the first order nuclei, which receive their primary afferent fibers from ascending pathways [Guillery RW (1995) Anatomical evidence concerning the role of the thalamus in corticocortical communication: a brief review. J Anat 187 (Pt 3):583-592], was significantly higher than that in the higher order nuclei. While moderate expression was also detected in the adult dorsal thalamus, the expression in the first order nuclei was almost the same as that in the higher order nuclei. Thus, the in situ hybridization experiments indicated that the transient postnatal increase in the amount of growth-associated protein-43 mRNA, which was shown by the Northern blot analysis, was mainly attributed to enhanced expression in the first order nuclei during the postnatal period. This may be a molecular basis for environmentally induced modification of thalamocortical synapses.