Progesterone (P₄) and estradiol-17β (E₂) concentrations were measured in serum samples obtained from 23 captive and 23 free-ranging adult female Japanese black bears. We then determined the relationship between changes in these sex steroid hormones and pregnancy. In all captive bears, which included animals of both known and unknown reproductive status, serum P₄ concentrations were low from April to July, then tended to become higher after August. The levels then became much higher still in November and December, but returned to low levels in March. Serum P₄ concentrations in eight captive pregnant bears, which had parturitions the following spring, increased gradually from August (0.5-2.4 ng/ml) to October (0.9-3.6 ng/ml), and achieved significantly higher maximum levels in December (7.2-18.0 ng/ml). Thereafter, serum P₄ concentrations tended to decrease (3.5-6.4 ng/ml in January and 0.3-0.7 ng/ml in March). In all captive bears, serum E₂ concentrations varied from April to October but showed low levels in November and December, and became high in January. Serum E₂ concentrations in the eight pregnant bears were high in May (95.6-191.4 pg/ml) and varied from August to October (35.6-143.3 pg/ml). Subsequently, serum E₂ concentrations in December dropped to significantly lower minimum levels (5.3-11.9 pg/ml) and increased again in January (67.6-153.1 pg/ml). Among the free-ranging bears, the data on serum P₄ concentrations in eight bears led to expectations of pregnancy, whereas serum E₂ concentrations showed no distinct evidence related to pregnancy. These results, particularly in captive pregnant bears, indicate that a marked increase of P₄ in December might be accompanied by reactivation of the corpus luteum preceding implantation. Furthermore, changes in E₂ concentrations suggested the possibility that a decline in December and an increase in January are associated with implantation and parturition, respectively.