A method for the demonstration of the topographical distribution of phenobarbital at the cellular level in various tissues was established. Mice that had been exposed to various doses of phenobarbital by intraperitoneal injection were killed, and their tissues were fixed with 0.1 M phosphate buffer solution (pH 7.4) containing paraformaldehyde and glutaraldehyde. Thereafter, paraffin and frozen sections were made and stained by the indirect immunoperoxidase method using antisera obtained from commercial sources and used for the immunochemical assay of the blood level of phenobarbital in clinical medicine. A specific positive reaction was observed solely in testing the intoxicated tissues, and this reaction was inhibited when phenobarbital was added to the antisera. The minimal sensitivity of the positive reaction, which can be discerned by observing the stained slides macroscopically, was in the range of 10 mg/kg. Thus, the diagnosis of phenobarbital intoxication in the forensic autopsy can be made by immunohistochemistry. A positive reaction was found in various tissue cells, including nerve cells, myelin sheaths, glia cells, hepatocytes, cells of the alveolar and bronchial wall, epithelial cells of the distal part of the renal tubules, and so forth. Endothelial cells of the capillaries in all tissues gave a strong positive reaction. The immunocytochemical electron microscopy of the hepatocytes revealed that the positive reaction in the cytoplasm was located solely in the intraluminal space of the smooth endoplasmic reticulum. These results indicate some interesting aspects of the pharmacokinetics of phenobarbital in vivo. It is expected that the antisera, which are used widely for the assay of the blood concentration of various drugs (phenobarbital, amphetamines, morphine, and so forth), may be regarded as excellent reagents for immunocytochemistry. This clearly indicates that morphological evidence in toxicology, which had so far remained obscure, can be easily obtained by applying these antisera against various drugs.