The development of the olfactory and terminalis systems was studied in tissue from eight embryonic and early fetal specimens belonging to three species of baleen whales. In contrast to toothed whales, baleen whales, particularly in these ontogenetic stages, are much less specialized in nasal organ morphology. The nasal cavity and peripheral olfactory system are well developed and do not show signs of reduction. However, as in toothed whales, there is no trace of a vomeronasal organ or nerve. The terminalis neuroblasts can already be distinguished from the olfactory material in the embryonic period, and they form compact masses medial and caudal to the developing olfactory bulb. As in most prenatal toothed whales, there are two large intrameningeal terminalis ganglia. These are connected with the telencephalic wall by central rootlets and with the septal mucosa by fiber bundles running through the level of the future cribriform plate. Clusters of terminalis neuroblasts also lie near the septal mucosa and along the peripheral terminalis fiber bundles. The functional implications of the olfactory and terminalis systems in whales are discussed.