The feasibility of ultrahigh resolution optical coherence tomography (UHR OCT) to image ex vivo and in vitro brain tissue morphology on a scale from single neuron cells to a whole animal brain was investigated using a number of animal models. Sub-2-μm axial resolution OCT in biological tissue was achieved at different central wavelengths by separately interfacing two state-of-the-art broad bandwidth light sources (titanium:sapphire, laser, and a fiber laser light source, to free-space or fiber-based OCT systems, designed for optimal performance in the appropriate wavelength regions. The ability of sub-2-μm axial resolution OCT to visualize intracellular morphology was demonstrated by imaging living ganglion cells in cultures. The feasibility of UHR OCT to image the globular structure of an entire animal brain as well as to resolve fine morphological features at various depths in it was tested by imaging a fixed honeybee brain. Possible degradation of OCT axial resolution with depth in optically dense brain tissue was examined by depositing microspheres through the blood stream to various depths in the brain of a living rabbit. It was determined that in the 1100 to 1600-nm wavelength range, OCT axial resolution was well preserved, even at depths greater than 500 μm, and permitted distinct visualization of microspheres 15 μm in diameter. In addition, the OCT image penetration depth and the scattering properties of gray and white brain matter were evaluated in tissue samples from the visual cortex of a fixed monkey brain. © 2004 American Institute of Physics.