The structure and development of the brain are extremely difficult to study in free-ranging marine mammals. Here, we report measurements of total white matter (WM), total gray matter (GM), cerebellum (WM and GM), hippocampus, and corpus callosum made from magnetic resonance (MR) images of fresh, postmortem brains of the Atlantic white-sided dolphin (Lagenorhynchus acutus) imaged in situ (i.e., the brain intact within the skull, with the head still attached to the body). WM:GM volume ratios of the entire brain increased from fetus to adult, illustrating the increase in myelination during ontogeny. The cerebellum (WM and GM combined) of subadult and adult dolphins ranged from 13.8 to 15.0% of total brain size, much larger than that of primates. The corpus callosum mid-sagittal area to brain mass ratios (CCA/BM) ranged from 0.088 to 0.137, smaller than in most mammals. Dolphin hippocampal volumes were smaller than those of carnivores, ungulates, and humans, consistent with previous qualitative results assessed from histological studies of the bottlenose dolphin brain. These quantitative measurements of white matter, gray matter, corpus callosum, and hippocampus are the first to be determined from MR images for any cetacean species. We establish here an approach for accurately determining the size of brain structures from in situ MR images of stranded, dead dolphins. This approach can be used not only for comparative and developmental studies of marine mammal brains but also for investigation of the potential impacts of natural and anthropogenic chemicals on neurodevelopment and neuroanatomy in exposed marine mammal populations.