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Ketamine is a widely used pediatric anesthetic recently reported (C. Ikonomidou et al., 1999, Science 283, 70–74) to enhance neuronal death in neonatal rats. To confirm and extend these results, we treated four groups of PND 7 rats with seven sc doses, one every 90 min, of either saline, 10 mg/kg ketamine, 20 mg/kg ketamine, or a single dose of 20 mg/kg ketamine. The repeated doses of 20 mg/kg ketamine increased the number of silver-positive (degenerating) neurons in the dorsolateral thalamus to a degree comparable to previous results (Ikonomidou et al., 1999, Science 283, 70–74), i.e., 28-fold vs. 31-fold respectively. However, blood levels of ketamine immediately after the repeated 20 mg/kg doses were about 14 μg/ml, about seven-fold greater than anesthetic blood levels in humans (J. M. Malinovsky et al., 1996, Br. J. Anaesth. 77, 203–207; R. A. Mueller and R. Hunt, 1998, Pharmacol. Biochem. Behav. 60, 15–22). Levels of ketamine in blood following exposure to the multiple 10 mg/kg doses of ketamine or to a single 20 mg/kg dose ranged around 2–5 μg/ml; although these blood levels are close to an anesthetic level in humans, they failed to produce neurodegeneration. To investigate the mode of ketamine-induced neuronal death, coronal sections were stained with both Fluoro-Jade B (a green fluorescent stain selective for neurodegeneration) and DAPI (a blue DNA stain), as well as for caspase-3 (using an antisera labeled red with rhodamine). These histochemical results confirmed the developmental neurotoxicity of ketamine, demonstrated that Fluoro-Jade B (FJ-B), like silver methods, successfully stained degenerating neurons in neonatal rats, and indicated that ketamine acts by increasing the rate of neuronal apoptosis.