The developmental change of Mg2+ block of NMDA-induced response (INMDA) was investigated in the freshly dissociated nucleus tractus solitarii (NTS) neurons of the rats by the use of a nystatin-perforated patch-recording configuration. Mg2+ block was less obvious in a number of fetal NTS neurons than in the neurons of > 9 days after birth (P9) and became rapidly apparent by P3. Protein kinase C (PKC) modulators, such as staurosporine, H-7 and phorbol ester, did not clearly affect the generation of the voltage dependency of INMDA in immature rats. In addition, the facilitatory effect of glycine on the INMDA did not change in development. These evidences suggest that an appearance of the voltage dependency of INMDA in NTS neurons might be due to a developmental change in combinations of subunits composing the NMDA receptor and/or in the intracellular modulators of the INMDA other than PKC.