To clarify the endocrine mechanism involved in the short captivity stress in the water frog, Rana esculenta, the activity of 9-ketoreductase, the enzyme which converts prostaglandin E2 (PGE2) into prostaglandin F2 alpha (PGF2 alpha), and aromatase, which converts testosterone into oestradiol-17 beta, were studied. Adult male and female frogs were sacrificed 0, 1.5, 3, 6, 12, 24, 48, 72, 168 and 336 h after capture in the field. PGE2, PGF2 alpha, progesterone, testosterone, oestradiol-17 beta and corticosterone plasma levels were detected by RIA at each time point. 9-Ketoreductase (conversion of [3H]PGE2 into [3H]PGF2 alpha) and aromatase (conversion of [3H]testosterone into [3H]oestradiol-17 beta) activities in the brain, testis, ovary and interrenal were also determined at each time point. After capture, levels of plasma PGF2 alpha increased (male: 228%; female: 288%) and PGE2 decreased (male: 68%; female: 81%) at 1.5 h, oestradiol-17 beta increased (male: 399%; female: 425%) and testosterone decreased (male: 87%; female: 83%) at 6 h, and corticosterone increased (male: 421%; female: 426%) at 72 h. 9-Ketoreductase activity in the brain was enhanced at 1.5 h after capture (male: 249%; female: 262%); aromatase activity increased at 6 h in the testis (261%), ovary (273%) and interrenal (male: 227%; female: 267%). These results indicate that short captivity stress could induce an increase in plasma PGF2 alpha through activation of brain 9-ketoreductase. In turn, PGF2 alpha might enhance the levels of circulating oestradiol-17 beta through activation of gonadal and interrenal aromatase.