Elucidating the mechanism of toxicity is crucial in drug safety evaluations. TOXic Process Ontology (TXPO) systematizes a wide variety of terms involving toxicity courses and processes. The first version of TXPO focuses on liver toxicity. The TXPO contains an is-a hierarchy that is organized into three layers: the top layer contains general terms, mostly derived from the Basic Formal Ontology. The intermediate layer contains biomedical terms in OBO foundry from UBERON, Cell Ontology, NCBI Taxon, ChEBI, Gene Ontology, PATO, OGG, INOH, HINO, NCIT, DOID and Relational ontology (RO). The lower layer contains toxicological terms. In applied work, we have developed a prototype of TOXPILOT, a TOXic Process InterpretabLe knOwledge sysTem. TOXPILOT provides visualization maps of the toxic course, which facilitates capturing the comprehensive picture for understanding toxicity mechanisms. A prototype of TOXPILOT is available: https://toxpilot.nibiohn.go.jp