On August 10, 2026, the Organisation for Economic Co-operation and Development (OECD) posted a Guidance Document on Toxicokinetics to accommodate Testing of (Nano)particles that provides guidance and considerations around in vivo testing for toxicokinetics of simple-structured, manufactured, poorly soluble, and partially soluble spheroidal particles following oral and inhalation exposure. To reflect that both nano-sized and larger particles are included in the scope of the guidance document, OECD uses the term “nano” between parentheses, (nano)particles. OECD notes that the primary scope is also on inorganic (nano)particles, although some information relevant for organic materials is also included. OECD based the scope on what was considered feasible given the available scientific data at the time of writing. OECD states that it may make future updates or additions as new information becomes available and/or structured for the purpose of providing additional guidance, including for a wider range of materials.