We test the hypothesis that the New Zealand sea lion is physiologically better equipped for prolonged, continuous diving than other otariids (fur seals and sea lions) by measuring its blood volume, an important component of its oxygen storage. Mass, hematocrit, and plasma volume were measured and blood volume calculations were completed on 14 adult females and five juvenile females. Plasma volume was determined using the Evans blue dye dilution technique. Mean plasma volume for all subjects was 74 mL kg(-1). Mass-specific plasma volume was significantly higher in adult females (15.3%) than in juveniles (14.6%). Blood volume (150 mL kg(-1)) and hematocrit (51%) were not significantly different between adults and juveniles. The aerobic dive limit can be estimated by dividing the animal's oxygen stores by its metabolic rate. The estimated aerobic dive limit for adult animals was between 5.5 and 7.8 min, depending on the assumed metabolic rate. New Zealand sea lions have the highest blood volume yet reported for an otariid, which supports the hypothesis that they have a physiological capability suited to their unique diving behavior.