The measurement of circulating reproductive hormone levels provides a relatively noninvasive method for assessing reproductive status. We assessed seasonal changes in concentrations of progesterone and total estrogens from serum samples collected from live stranded (n = 199) and captive (n = 5) California sea lions (Zalophus californianus). Progesterone concentrations increased in the fall (September-November) in both pregnant and nonpregnant animals, with no significant difference associated with pregnancy. Progesterone concentrations were greater in pregnant animals in the spring (February-April) than in nonpregnant animals. Circulating estrogen concentrations in captive, nonpregnant sea lions increased in July and November, correlating with periods of estrus and implantation. These data provide a baseline for studying reproduction in California sea lions.