We used immunoblot analysis to investigate the phosphorylation of alpha-amino-3-hydroxy-5-methyl-4-isoxazole proprionic acid (AMPA)-receptor glutamate receptor-1 (GluR1) and related protein kinases in rat hippocampus on postnatal days (PND) 1-28. Total GluR1 expression increased up to PND 9, and stayed high thereafter. The proportions of the forms of GluR1 phosphorylated at serines (S) 845 and S831, which were both high at birth, decreased at different rates: phosphorylated S831 decreased rapidly after PND 7, and was almost zero after PND 21, while phosphorylated S845 decreased slowly, and after PND 14 stayed at one third of its PND 1 level. The expression patterns of cyclic AMP-dependent protein kinase (PKA) and protein kinase C (PKC), but not that of calcium/calmodulin-dependent protein kinase II, were similar to those of the forms of GluR1 phosphorylated at S845 and S831, respectively. Thus, the status of GluR1 phosphorylation, through PKA and PKC modulation, may contribute to the development of hippocampal synaptic plasticity.