We examined the effect of continuous intracerebroventricular (i.c.v. ) administration of pentobarbital on the level of N-Methyl-D-aspartate receptor NR1 subunit mRNAs in different regions of rat brain by in situ hybridization histochemistry. Animals were rendered tolerant to pentobarbital by continuous i.c.v. infusion (300 microgram/10 microliter/h for 6 days) through pre-implanted cannulae connected to osmotic mini-pumps, and dependent, by abrupt withdrawal from pentobarbital. NR1 subunit mRNA level was significantly decreased in the entorhinal cortex, cingulate, caudate-putamen, septum, and the CA1 of hippocampus in pentobarbital withdrawal rats. The level of NR1 mRNAs was decreased in only CA1 of hippocampus in pentobarbital tolerant rats. These results indicate that withdrawal from i.c.v. infusion with pentobarbital decreases in NR1 subunit expression in the rat brain, suggesting that changes in expression of NR1 subunit may contribute to the development of withdrawal from pentobarbital.