The effects of beta-endorphin and its receptor antagonist, naloxone, on corticosterone and cortisol production in male and female Triturus carnifex were studied in vivo and in vitro. In the in-vivo experiment, the animals were injected s.c. with beta-endorphin and/or naloxone, and killed after 15, 30, 90 and 360 min. In the in-vitro experiment, interrenal tissues, with and without added pituitary, were incubated with beta-endorphin and/or naloxone for 15, 30, 60 and 120 min. The data obtained in vivo and in vitro from males and females were in agreement. Treatment with beta-endorphin caused a significant decrease in corticosterone and cortisol release, while naloxone induced an increase in the two corticosteroids at the same times as the decrease caused by beta-endorphin. The combined beta-endorphin plus naloxone treatment did not change corticosterone and cortisol levels. These results suggest that, in Triturus carnifex, opioids are involved in the regulation of the hypothalamo-pituitary-interrenal axis. In particular, the in-vitro results indicate a direct effect of opioids on interrenal steroidogenesis.