Because of variations in enzymology and functional capacity of neonatal organ systems, care must be taken when interpreting any changes in clinical chemistry values when using standard adult reference ranges. Although published reference ranges are provided, based on the available research in current literature it is recommended-that these ranges are used only as guidelines, owing to the lack of standardization of reference intervals among reference laboratories. This article is not intended to provide a fully comprehensive review of all hematologic and biochemical changes that occur from birth to 6 months of age, but to help provide a practical guideline for interpretation and useful diagnostic information in determining the state of health or cause of illness in a young dog or cat.