NF-kappaB was shown previously to regulate apoptotic cell death processes in various experimental systems. However, its role in controlling teratogen-induced cell death has not been established yet. Therefore, the objective of the present study was to explore the involvement of the p65 subunit of NF-kappaB in the response of mouse embryonic fibroblasts (MEFs) to heat shock, using p65 knockout (p65-/-) cells. Indeed, we found p65-/- MEFs to be more susceptible to the exposure to heat shock, as compared with wild-type (WT) MEFs, as they demonstrated a more prominent decrease in cell survival and proliferation as well as the appearance of cells undergoing apoptotic cell death. These heat-shock-induced effects were preceded by a decrease in p65 expression in WT cells, which was accompanied by a decrease in IkappaBalpha expression in WT MEFs, while disappearing completely in p65-/- MEFs and accordingly, by an increase in p-IkappaBalpha expression in both cell lines, which was found to be more prominent in p65-/- MEFs. Interestingly, the heat shock-induced decrease in p65 expression was accompanied by an increase in HSP70 expression in both cell lines. However, it was again found to be more prominent in p65-/- MEFs. Taken together, our results suggest a protective role for the p65 subunit of NF-kappaB in mechanisms underlying the response of embryonic cells to heat shock.