ABSTRACT Leptin is an obese gene product, and leptin-deficient ob/ob mice develop hyperphagia and reduced locomotor activity. Leptin is thought to be related to brain development, because leptin receptors are widely expressed in the brain, and because brain weight as well as brain protein and DNA contents were reduced in adult ob/ob mice. In this study, we investigated the effect of leptin on the fetal cingulate cortex, since the leptin receptor is expressed in the neurons of the cingulate cortex, which is involved in emotion as well as in sensory, motor, and cognitive processes. The ob/ob fetuses had more pycnotic cells than wild-type fetuses in the cingulate cortex at embryonic day (E) 18. Many pycnotic cells were observed in the intermediate zone of the cingulate cortex. Most cells observed in this area were neuronal lineage cells, while few undifferentiated cells and oligodendrocyte precursor cells were found. At E18 there was no significant difference in the rostrocaudal length of the corpus callosum, which contains the neuronal projection from the cingulate cortex, between ob/ob and wild-type fetuses. We also showed that the length of the cerebrum was greater and the width of the cerebrum and cerebellum were lesser in ob/ob fetuses than in wild-type at E16. These results suggest an increased cell death in neuronal lineage cells in the intermediate zone of the cingulate cortex in leptin-deficient ob/ob mice. Leptin deficiency may also alter the gross morphology of the brain in development, but not the formation of the corpus callosum.