Gonadotropin-inhibitory hormone (GnIH) is a neurohormone that suppresses reproduction by acting at both the brain and pituitary levels. In addition to the brain, GnIH may also be produced in gonads and can regulate steroidogenesis and gametogenesis. However, the function of GnIH in gonadal physiology has received little attention in fish. The main objective of this study was to evaluate the effects of peripheral sbGnih-1 and sbGnih-2 implants on gonadal development and steroidogenesis during the reproductive cycle of male sea bass (Dicentrarchus labrax). Both Gnihs decreased testosterone (T) and 11-ketotestosterone (11-KT) plasma levels in November and December (early- and mid-spermatogenesis) but did not affect plasma levels of the progestin 17,20ß-dihydroxy-4-pregnen-3-one (DHP). In February (spermiation), fish treated with sbGnih-1 and sbGnih-2 exhibited testicles with abundant type A spermatogonia and partial spermatogenesis. In addition, we determined the effects of peripheral Gnih implants on plasma follicle-stimulating hormone (Fsh) and luteinizing hormone (Lh) levels, as well as on brain and pituitary expression of the main reproductive hormone genes and their receptors during the spermiation period (February). Treatment with sbGnih-2 increased brain gnrh2, gnih, kiss1r and gnihr transcript levels. Whereas, both Gnihs decreased lhbeta expression and plasma Lh levels, and sbGnih-1 reduced plasmatic Fsh. Finally, through behavioral recording we showed that Gnih implanted animals exhibited a significant increase in diurnal activity from late spermatogenic to early spermiogenic stages. Our results indicate that Gnih may regulate the reproductive axis of sea bass acting not only on brain and pituitary hormones but also on gonadal physiology and behavior.