The effects of intraperitoneal injection of gonadotropin-releasing hormone (GnRH) alone, naloxone (NAL) alone, or in combination on plasma levels of follicle stimulating hormone (FSH) and luteinizing hormone (LH) was studied in 4- to 5-wk-old male pigs. GnRH (1 μg kg−1) effectively stimulated (P < 0.05) secretion of both gonadotropins whereas NAL (1 and 10 mg kg−1) stimulated only FSH secretion (P < 0.05). There was no interaction between GnRH and NAL on gonadotropin release. These results suggest that endogenous opiates are involved in the regulation of FSH secretion but not LH secretion in the immature male pig. Key words: Follicle-stimulating hormone, luteinizing hormone, naloxone, gonadotropin-releasing hormone, male pig