A general technique is described for using slide-mounted unfixed tissue sections to characterize and visualize drug and neurotransmitter receptors in brain or other tissues. The preparation of material, from fresh frozen, unfixed brain to dried sections securely attached to slides, is described in detail. The tissue can be kept intact during incubation at varying temperatures in solutions containing radiolabeled ligand, ions, buffers, and allosteric effectors. Strategies are described for determining optimal stereospecific binding with highest signal-to-noise ratios and for determining that a “meaningful” receptor is being studied. Dry formaldehyde fixation by vapors from heated paraformaldehyde preserves the tissue quality and traps the ligand near its site on the receptor, permitting subsequent histological processing through alcohols, solvents, and aqueous media, including liquid nuclear track emulsion. Visualization of [3H]naloxone- or [3H]enkephalin- labeled opiate receptor distributions in rat and human brains is achieved by tritium-sensitive film or by classical “wet” emulsion autoradiography. The advantages of the film include its ease of use and the ability to quantify receptor density by densitometry which can be computer-assisted. The advantage of the emulsion is the greater resolution and the concomitant appearance of morphology in cell-stained sections. Examples of correlations of opiate receptor distributions which underlying cytoarchitecture illustrate the potential for receptor localization studies.