To maintain adenosine triphosphate (ATP) production, muscles working under anaerobic conditions produce lactic acid (C 3H 6O 3), which has been linked to muscle fatigue and acidosis. Marine mammals often dive for extended periods of time under hypoxic conditions and have adapted to cope with the build up of lactic acid produced during occasional, very long dives. Hooded (Cystophora christata) and harp (Pagophilus groenlandicus) seals offer a unique comparison of muscle physiology because hooded seals prefer to forage using long, deep dives (avg. 450 m, 16 min) while harp seals prefer to forage using short, shallow dives (avg. 95 m, 5 min). Our aim was to compare the ability of hooded and harp seals to operate under hypoxic conditions through the comparison of acid buffering capabilities (β) and myoglobin (Mb) content in cardiac and skeletal muscle. For acid buffering, muscle tissue was homogenized in a saline solution and then titrated with NaOH while measuring changes in pH levels. Myoglobin content was determined spectrophometrically from reduced muscle supernatant. In both species, there was a significant increase in the Mb content of skeletal and cardiac muscle between adults and neonates and an age-related increase in β in skeletal muscle. However, there was no significant difference in β in the cardiac tissue between adults and neonates in either species. This implies that while neonatal cardiac muscle may be able to withstand lactic acid build-up at birth, skeletal muscle buffering capacity and oxygen store capacity needs to develop the as animals age and may dictate juvenile diving and feeding behaviors.