Although stimulation of N-methyl-D-aspartate receptors or voltage-dependent calcium channels induces both the activation of c-fos and brain-derived neurotrophic factor (BDNF) genes, it is not certain how the activation of these genes is related. Using primary cultures of rat hippocampal neurons, we found that exposing the cells to cycloheximide allowed subsequent activation of BDNF mRNA expression, although activation of AP-1 DNA-binding activity resulting from the c-fos induction was abolished. Super-induction of BDNF gene was also caused by cycloheximide. The estimated half-life of BDNF mRNA was approximately 2.5 hrs, which was almost identical to that of c-fos mRNA. These results indicate that nascent AP-1 is not required for the activation of BDNF gene, leading to the notion that the BDNF gene can be activated by Ca2+ signals as an immediate early gene.