Previous studies have shown that ionizing radiation-induced cell death in the developing brain has morphological characteristics of apoptosis and is associated with internucleosomal DNA fragmentation. In the present study, we have observed c-Jun induction in cells sensitive to ionizing radiation during the whole process of radiation-induced cell death, and that this expression is accompanied by modifications in the composition of AP-1 complexes: c-Jun/AP-1 activity is highly increased whereas Jun D/AP-1 is slightly decreased. These results show that c-Jun expression and c-Jun/AP-1 activity are induced in the developing brain following ionizing radiation.