Results of experiments in which regional neuronal activity is revealed by a 2-[3H]deoxy-D-glucose (3H-2-DG)-paraffin section-emulsion autoradiography method are described. The trigeminal pathway of freely behaving mice was activated differentially by selective patterns of whisker removal. One hour after injection of concentrated 3H-2-DG, the animals were perfused systemically with a periodate/lysine/paraformaldehyde mixture (McLean, I. W., and P. K. Nakane (1974) J. Histochem. Cytochem. 22: 1077–1083), the brains were embedded in paraffin, and serial sections were taken and coated with emulsion for autoradiography. Diffusion of the isotope out of the tissue was assessed visually and by liquid scintillation counting. While substantial loss of 3H isotope into the embedding fluids (about 95%) was found, the scintillation counts and the autoradiograms showed good fixation of the isotope in situ, no evidence of isotope movement into the emulsion, and no gradients of diffusion in the sectioned material. Patterns of regional labeling were similar to those reported from brains prepared by conventional 2-[14C]deoxy-D-glucose (14C-2-DG) autoradiography; for instance, auditory and vestibular pathways in the brainstem were heavily and specifically labeled. Trigeminal structures associated with the intact (stimulated) whiskers were labeled relatively heavily, indicating that label uptake is specific with respect to neuronal activity. In the cortex, the patterns of label corresponded directly and precisely to those barrels known to receive inputs from the intact whiskers. Distribution of silver grains in the cortex and in the brainstem was correlated directly with neuronal profiles, including processes, some of which were identified by means of a Nissl counterstain. Clearly, this approach offers considerable technical advantages, in particular, the ease with which the histological material is prepared. The resolution of the autoradiograms and the quality of the histology are excellent.