Feline infectious peritonitis (FIP) is a highly fatal viral disease of cats with worldwide prevalence. Recently, several antiviral agents have shown promise for the treatment of cats with FIP. Despite these findings, there are no current FDA-approved legal treatments for FIP, leaving many owners to turn to unlicensed antiviral drugs purchased online. As a result of the ongoing COVID-19 pandemic, several antiviral agents have been recently approved for oral use in human patients. One of these antivirals is the nucleoside analog EIDD-2801 (Molnupiravir). EIDD-2801 results in hypermutation of the developing viral RNA, effectively terminating viral replication for a number of different RNA viruses. This antiviral agent has been developed for oral use in human patients at home. The utilization of EIDD-2801 in human medicine may facilitate veterinary use. An orally administered antiviral compound would be advantageous for client-administered treatment of cats with FIP. We will determine an effective oral dose for EIDD-2801 and assess the effectiveness of this compound in home-based treatment of client-owned cats with naturally occurring, effusive FIP.