An animal-borne blood sampler with data logging functions was developed for phocid seals which collected two blood samples for the comparison of endocrinological/biochemical parameters under two different conditions. The sampler can be triggered by preset hydrostatic pressure, acceleration (descending or ascending), temperature and time, and also manually by light. The sampling was reliable with >78% successful attempts to collect blood samples. Contamination of fluids in the tubing to the next blood sample was <1%, following the prior clearance of the tubing to a waste syringe. In captive harbor seals (Phoca vitulina) the automated blood sampling method was less stressful than direct blood withdrawal as plasma levels of stress hormones were lower in the former (p<0.05 for ACTH and p=0.078 for cortisol). HPLC analyses showed that both cortisol and cortisone were circulating in seal blood. Using the sampler, plasma levels of cardiovascular hormones, atrial natriuretic peptide (ANP), arginine vasopressin (AVP), and angiotensin II (AngII), were compared in grey seals (Halichoerus grypus), between samples collected when the animals were on land and in the water. HPLC analyses determined that [Met¹²] ANP (1-28) and various forms of angiotensins (AngII, III and IV) were circulating in seal blood. Although water immersion profoundly changes the plasma levels of cardiovascular hormones in terrestrial mammals, there were only tendencies towards an increase in ANP (p=0.069) and a decrease in AVP (p=0.074) in the seals. These results suggest that cardiovascular regulation in phocid seals may have undergone adaptation during evolution of the carnivore to a semi-aquatic lifestyle.