Polybrominated diphenyl ethers (PBDEs), one class of flame retardants used to suppress or inhibit the risk of fire, are regularly found in the environment and in human milk. The present study shows that neonatal exposure to a widely, environmentally found PBDE, 2,2′,4,4′,5-pentaBDE (PBDE 99), can induce developmental neurotoxic effects, such as changes in spontaneous behavior (hyperactivity), effects that are dose-response related and worsen with age. These changes are seen in C57/Bl mice of both sexes. Neonatal C57/Bl male and female mice were orally exposed on day 10 to 0.4, 0.8, 4.0, 8.0, or 16 mg PBDE 99/kg body weight. Spontaneous behavior (locomotion, rearing, and total activity) was observed in two-, five-, and eight-month-old mice. The behavior tests showed that the effects were dose-response and time-response related for both male and female mice. The observed developmental neurotoxic effects seen for PBDE 99, in C57/Bl mice, are similar to effects seen for 2,2′,4,4′-tetraBDE (PBDE 47), PBDE 99, 2,2′,4,4′,5,5′- hexaBDE (PBDE 153), 2,2′,3,3′,4,4′,5,5′,6,6′-decaBDE (PBDE 209) and for certain PCBs, in male NMRI mice. Furthermore, the effects of PBDEs appear to be as potent in female mice as in male mice, and as potent in C57/Bl mice as in NMRI mice, concerning developmental neurotoxicity.