Background and Purpose: Most therapeutic agents are administered intravenously (IV) in clinical settings, nevertheless in most preclinical studies with neonatal rodents, intraperitoneal administration (IP) is a common route to deliver drugs. However, it remains unclear whether IP is truly an acceptable alternative for intravenous (IV) injection in preclinical studies. IV injection in neonatal rodents has been believed technically impossible, but recently we have established a method to administer via femoral vein in neonatal mice. The objective of our study is to clarify the differences in the therapeutic effects of drugs after an IP or IV injection in animals with brain injury. Methods: Mice were subjected to hypoxic-ischemic (HI) insult at postnatal day-8. Dexamethasone was administered either IP or IV at 24 h before HI insult. MK-801, an N-methyl-D-aspartate receptor antagonist, was administered either IP or IV just before HI insult. The % loss of cerebral hemisphere was analyzed after one week. Results: IP and IV administration of dexamethasone attenuated the brain injury to a similar degree (Dexamethasone-IP, 3.5 ± 0.9 % and Dexamethasone -IV, 7.7 ± 1.7 % vs. PBS-IV, 16.9 ± 3.2 %). IP administration of MK-801 attenuated brain injury, whereas IV administration of MK-801 did not (MK-801-IP, 10.2 ± 2.9 % and MK-801-IV, 28.6 % ± 7.6 % vs. PBS-IV, 32.8 ± 5.5 %). Conclusions: This study demonstrated that the administration route influences the therapeutic effects of drugs. Therefore, a preclinical study may need to be performed using the optimal administration route which is designed to be used in a clinical setting.