This thesis investigated the influence of seasonal changes in ambient conditions on electrolyte status in outdoor housed resting dogs and horses over multiple months. While the effect of season on various hematological and biochemical parameters is well recognized in human and veterinary medicine, little is known about its impact on electrolyte status in these animals. These investigations revealed that dogs fed a commercial kibble and horses provided with ad libitum forage maintained their serum and plasma electrolyte concentrations with minimal monthly variation when hydrated, acclimatized to their environment, and provided with free-choice shelter access. However, for animals that are exercising, not adequately hydrated, or not acclimatized to their environment, seasonal variations in electrolyte concentrations may have significant biological implications. Hence, animal practitioners may need to consider seasonal or monthly variations when interpreting electrolyte concentrations from different periods of the year or when caring for animals at risk of electrolyte imbalance.