Marine mammal researchers and veterinarians evaluate the health of individuals and populations by using observed ranges for blood values. The goal of this study was to establish observed ranges for dependent (0–4 week) and weanling (>4 week) harbor seal (Phoca vitulina) pups that were admitted for rehabilitation at the Marine Animal Lifeline in Portland, Maine from 1998–2003 and to record changes in hematologic and biochemical values as they convalesced. Blood values, measured both at admission to and release from rehabilitation, were quantitatively compared between these two groups. Of the 29 blood analytes measured in this study between dependent and weanling pups upon admission to rehabilitation, 14 were significantly different including white blood cell (WBC), red blood cell (RBC), hemoglobin (Hb), hematocrit (HCT), platelets, lymphocytes, monocytes, potassium, chloride, CO2, total bilirubin, calcium, alanine transaminase (ALT), and alkaline phosphatase. Between admission and release, 14 analytes had significantly changed for dependent pups including RBC, Hb, HCT, mean cell volume (MCV), mean corpuscular hemoglobin (MCH), platelets, lymphocytes, monocytes, potassium, blood urea nitrogen (BUN), total protein, total bilirubin, alkaline phosphatase, and ALT. Between admission and release, 10 analytes had significantly changed for weanlings including RBC, MCV, neutrophils, eosinophils, potassium, chloride, anion gap, total protein, total bilirubin, and calcium. At the time of release, six parameters were significantly different between pups admitted as dependent and those admitted as weanlings including MCV, neutrophils, eosinophils, total protein, globulin, and albumin.