Leukemia inhibitory factor contributes to the self-renewal of neural stem cells in the forebrain. Although the existence of endogenous leukemia inhibitory factor in the brain parenchyma has been controversial, the cerebrospinal fluid is known to be another source of leukemia inhibitory factor. No reports of the measurement of leukemia inhibitory factor concentrations in the cerebrospinal fluid, however, exist. In the present study, we determined the leukemia inhibitory factor concentration in cerebrospinal fluid, amniotic fluid, and sera of embryos and dams in mice by enzyme-linked immunosorbent assay. The leukemia inhibitory factor concentrations were found to be constitutively high in the cerebrospinal fluid from embryonic day 11 to embryonic day 17, with a peak on embryonic day 13 and embryonic day 14. These findings correspond to the timing of cortical neuron production in mouse cerebrum.