The opioid peptide β-endorphin (β-EP) is known to play modulatory role in vertebrate reproduction. The objective of the present study was to determine the influence of β-EP and the opioid receptor antagonist naltrexone (NALT) on follicular development in normal, and stressed female tilapia Oreochromis mossambicus. Administration of 4 µg β-EP, but not 0.5 or 1.5 µg β-EP daily for 22 days caused a significant reduction in the mean GSI, HSI, and number of Stage IV (vitellogenic) follicles compared to experimental controls during the prespawning phase of the ovarian cycle. Furthermore, the Stage V (vitellogenic) follicles were completely absent during the prespawning phase concomitant with the significantly lower serum levels of estradiol and cortisol in 4 µg β-EP treated fish compared to experimental controls. On the other hand, exposure of the fish to mild acute stressors for 22 days caused changes in the ovary similar to that of high dose of β-EP, whereas administration of NALT attenuated these effects. The results reveal that β-EP exerts inhibitory effect on the vitellogenic follicular growth through the suppression of ovarian steroidogenesis. The study provides evidence for the opioidergic mediation of the ovarian stress response for the first time in fish. J. Exp. Zool. 319A:156–165, 2013. © 2013 Wiley Periodicals, Inc.