White matter of the preterm brain is highly vulnerable to injury from hypoxia/ischemia and infection. However, while many premature infants experience multiple episodes of hypoxia in the course of neonatal intensive care, the effect of recurrent hypoxia on white matter injury is relatively unexplored. We previously demonstrated that sublethal exposure to hypoxia in the very immature rat at postnatal day (P)3 modifies subsequent AMPA receptor activity in developing oligodendrocytes at P5; and that hypoxia at P3, then repeated at P5, alters subsequent myelin basic protein expression (Follett et al, SFN Abstracts, 2002). Rats subjected to a single episode of hypoxia at either P3 or P5 do not show changes in white matter maturation by immunocytochemistry. In contrast, rats subjected to hypoxia (1h, 6%) at P3 and again at P5 show significantly fewer O1 expressing immature oligodendrocytes in white matter (n=8, p<0.001), as compared with littermate controls (n=6). A mean of 52.3±14.7% of white matter cells expre...