PURPOSE: Apoptosis is active cell death which plays an important role in developing normal tissues. Various conditions such as genetic defects, drugs, ischemia or infections are known to induce apoptosis. We studied the effect of maternal infection on fetal brain development during pregnancy. METHODS: We treated 46 C3H pregnant mice with lipopolysaccharide (LPS) or phosphat-buffered saline and observed the changes in apoptosis and expression of bcl-2, bcl-xS, bax. The fetal brain tissues were removed 1-48 hours after LPS treatment. The number of apoptosis per 100 neurons and glial cells was counted in H&E stained tissue and was analyzed statistically. Immunohistochemical staining with primary antibodies of bcl-2, bcl-xS, bax was done and their expression was classified by the degree of staining. RESULTS: The number of apoptosis was increased significantly in both neurons and glial cells of LPS-treated group and its degree of staining was more remarkable in glial cells. Immunohisto chemistry for bcl-2, bcl-xS, bax oncoprotein revealed mildly decreased expression of bcl-2 and markedly increased expression of bax in both neurons and glial cells, but it was more remarkable in glial cells. Immunochemistry for bcl-xS revealed no expression in neurons and minimal expression of bcl-xS in glial cells in both study groups. CONCLUSOIN: We observed an increase in the number of apoptosis, mildly decreased expression of bcl-2 and markedly increased expression of bax in both neurons and glial cells of fetal brain after treating pregnant mice with LPS. Maternal infection during pregnancy may have profound effects on developing fetal brain.