Cortisol, a regulator of both energy intake and storage, changes with an animal's physiological state and is often used as an index of body fitness and stress response. To obtain a baseline concentration of cortisol in spotted seals, we analyzed serum cortisol concentrations using electro-chemiluminescence immunoassay in captive spotted seals (Phoca largha) (individual seals could be sampled multiple times with 64 cortisol samples from females and 81 from males) of different ages and in different reproductive seasons from the Liaodong Bay colony. Serum cortisol concentration first dropped steadily from 359 (334 to 463) nmol/L at 1 year of age to 254 +/- 81 nmol/L at 6 years of age, and then increased steadily to 387 +/- 55 nmol/L at age 12. This cortisol concentration pattern appears to experience a nadir concomitant with sexual maturity. We found significant serum cortisol concentration differences in the varying reproductive seasons, with cortisol concentration in the pre-breeding season (November to January; 341 +/- 88 nmol/L) and the breeding-molt season (February to March; 355 +/- 108 nmol/L) higher than in the other season (April to October; 212 [155 to 267] nmol/L; because the data for the other season is skewed, the interquartile range was presented). We did not detect serum cortisol concentration differences between male and female seals or between immature and mature seals. The present study provides a normal serum cortisol concentration profile related to age, sex, and reproductive season for captive spotted seals from the Liaodong Bay colony, which can provide helpful reference material for wild counterparts of this polar species.