Feline infectious peritonitis (FIP) is a fatal viral disease that affects cats around the world. The disease results in accumulation of fluid and inflammation in various body organs. Although there have been advances in research investigating new treatments for FIP which show great promise, both remain years away from being registered for veterinary use. A commonly used and available drug used for preventing malaria in people, mefloquine, was found to inhibit FIPV replication in laboratory cell cultures. Through Winn Feline Foundation funding support (2016[WINN_16_023] and 2019 [WINN_19_027]), our group investigated aspects of mefloquine’s in-vitro pharmacokinetic profile using feline hepatic metabolism enzymes (microsomes) and developed and validated an assay to quantify mefloquine concentrations in feline plasma. The way in which mefloquine was metabolized in these in vitro studies and its high level of binding to feline plasma proteins in plasma from clinically normal cats and those with feline infectious peritonitis, makes it unlikely that the drug will have delayed elimination in cats, which is safer for its prolonged use. Investigations into the way in which mefloquine behaves after oral administration including its safety profile in clinically normal cats (2019), found the drug to be well absorbed and safe to use. This investigation will use the drug on cats with naturally occurring FIP to determine its effectiveness in improving their well-being and longevity.