The molecular events responsible for impairments in cognition following mild traumatic brain injury are poorly understood. Neurotrophins, such as brain-derived neurotrophic factor (BDNF), have been identified as having a role in learning and memory. We have previously demonstrated that following experimental brain trauma of moderate severity (2.0-2.1 atm), mRNA levels of BDNF and its high-affinity receptor, trkB, are increased bilaterally in the hippocampus for several hours, whereas NT-3 mRNA expression is decreased. In the present study, we used in situ hybridization to compare BDNF, trkB, NT-3, and trkC mRNA expression in rat hippocampus at 3 or 6 h after a lateral fluid percussion brain injury (FPI) of mild severity (1.0 atm) to sham-injured controls at equivalent time points. Mild FPI induced significant increases in hybridization levels for BDNF and trkB mRNAs, and a decrease in NT-3 mRNA in the hippocampus. However, in contrast to the bilateral effects of moderate experimental brain injury, the present changes with mild injury were restricted to the injured side. These findings demonstrate that even a mild traumatic brain injury differentially alters neurotrophin and neurotrophin receptor levels in the hippocampus. Such alterations may have important implications for neural plasticity and recovery of function in people who sustain a mild head injury.