The physiological conditions that swell mammalian neurons are clinically important but contentious. Distinguishing the neuronal component of brain swelling requires viewing intact neuronal cell bodies, dendrites, and axons and measuring their changing volume in real time. Cultured or dissociated neuronal somata swell within minutes under acutely overhydrated conditions and shrink when strongly dehydrated. But paradoxically, most central nervous system (CNS) neurons do not express aquaporins, the membrane channels that conduct osmotically driven water. Using 2-photon laser scanning microscopy (2PLSM), we monitored neuronal volume under osmotic stress in real time. Specifically, the volume of pyramidal neurons in cerebral cortex and axon terminals comprising cerebellar mossy fibers was measured deep within live brain slices. The expected swelling or shrinking of the gray matter was confirmed by recording altered light transmittance and by indirectly measuring extracellular resistance over a wide osmotic range of -80 to +80 milliOsmoles (mOsm). Neurons expressing green fluorescent protein were then imaged with 2PLSM between -40 and +80 mOsm over 20 min. Surprisingly, pyramidal somata, dendrites, and spines steadfastly maintained their volume, as did the cerebellar axon terminals. This precluded a need for the neurons to acutely regulate volume, preserved their intrinsic electrophysiological stability, and confirmed that these CNS nerve cells lack functional aquaporins. Thus, whereas water easily permeates the aquaporin-rich endothelia and glia driving osmotic brain swelling, neurons tenatiously maintain their volume. However, these same neurons then swell dramatically upon oxygen/glucose deprivation or [K+]0 elevation, so prolonged depolarization (as during stroke or seizure) apparently swells neurons by opening nonaquaporin channels to water.