Feline infectious peritonitis (FIP) is an important disease characterized by granulomatous changes in various organs including the ocular and central nervous system. FIP infection is referred as the best candidate for antiviral drug development studies due to the ineffectiveness of vaccines and being a cause of high mortality. The most important antiviral drugs developed against RNA viruses are nucleoside analogues such as GS-441524. In this study, GS-441524 was used in the treatment of three naturally infected cats with ocular lesions diagnosed with effusive FIP. The treatment of cats was planned as GS-441524 (5 mg / kg, SC, daily) administration for 12 weeks along with fluid therapy on the admission day. The treatment process included follow-ups with clinical and laboratory examinations on the admission 5^th, 15^th, 30^th, 60^th, 90^th and 120^th days. On the 5^th day, clinical signs including ocular abnormalities and on the 30^th day, laboratory findings were significantly improved in all 3 cats. While the cats were expected for monthly follow-ups, it was learned that the cats died on the 58^th, 60^th and 62^nd days. It was concluded that GS-441524 (5 mg/kg/day, SC) has been proven to be beneficial for improving clinical, laboratory findings and animal welfare. However, it was evaluated that clinical studies involving more cases are required in order to determine the clinical efficacy of nucleoside analogues and investigate the causes of deaths occurred on the 58^th, 60^th and 62^nd days of the treatment period with no symptoms that would grab the attention of the owners.