The complete blood count (CBC), used to evaluate number of red blood cells and white blood cells in the animal’s system, is a commonly performed diagnostic test in veterinary medicine. Elevated white blood counts usually are indicative of infection and animals with elevated white blood count verified by CBC usually are treated with antibiotics. However, non-disease-related conditions have been associated with elevation in white blood count. Anecdotal evidence suggests that the onset of estrus, or heat, may be associated with elevation in white blood count and especially in elevation of one type of white blood cell, the eosinophil. This study will track white blood cell number and type in the weeks before estrus begins and over the estrous cycle to determine if a consistent alteration in white blood cell occurs in normal dogs at this time. This will allow clinicians to more accurately interpret CBC results near the time of estrus in bitches and minimize unnecessary use of antibiotics.