Chronic kidney disease is common in geriatric cats. Clinical signs include increased drinking and urination, decreased appetite, weight loss and vomiting. Decreased appetite can lead to weight loss, muscle weakness, and poor quality of life. Several studies have documented the value of specially formulated diets in the management of kidney disease. Therefore it is important for these patients to maintain their appetite and food intake. Mirtazapine (Remeron®) was introduced to human medicine as an antidepressant; however it has attracted interest in veterinary medicine because of several desirable side-effects, namely its significant anti-nausea, anti- vomiting, and appetite stimulating properties. Mirtazapine doses for cats have been adapted from human medicine; however no studies on how the drug is processed in the body of cats have been performed to verify this information. In order to provide accurate dosing recommendations for best effect and to avoid side-effects, it is necessary to investigate how the drug is processed in cats. The expected outcome of this study will be an understanding of the best dose and frequency of administration of mirtazapine to young and old cats, and cats with kidney disease. The study will also document how well the drug works as an appetite stimulant and anti-emetic in cats with kidney disease. The results of these studies will allow veterinarians to improve the quality of life of cats with chronic kidney disease.