The glycoprotein hormone family consists of follicle-stimulating hormone (FSH; GTH1), luteinizing hormone (LH; GTH2), and thyroid-stimulating hormone (TSH), which are secreted by the pituitary gland in all mammalian species, and chorionic gonadotropin, which is secreted by placental trophoblast cells in primates and equids. These hormones consist of non-covalently associated α-, β- subunits. Within a species, the amino acid sequence of α-subunit is identical across all glycoprotein hormones and is encoded by a single gene. The αβ dimer is the active form of the hormone, and biological specificity is conferred by the β-subunit. Both of α and β subunit of eel FSH has two N-glycosylation sites (α-subunit: Asn56 and Asn79; β -subunit: Asn5 and Asn22, respectively). In the present study, we constructed deglycosylated mutants at single and double sites in each subunits of eel FSH for identification of Asn linked oligosaccharides' biological role. Mutant cDANs were cloned into pcDNA3 expression vector and transiently transfected into CHO suspension cells. The quantity of rec-eelFSHs were quantified by sandwich ELISA system, using monoclonal antibodies produced in our lab. The wild type rec-FSH protein was detected at the predicted molecular weight of 34 kDa by western blot. The molecular weight of deglycosylated mutants at single site decreased with about 4 kDa and of mutants at double sites decreased with 8 kDa. After PNGase treatment in the rec-eel FSH proteins, molecular weight also decreased to 7-8 kDa. We generated stably parental cell lines, engineered to express a β-arrestin 2EA fusion protein, expressing eel FSHR and C-terminal deleted mutant. 2 out of 5 receptor cells each were selected by G-418 and we tested these cell lines in a receptor functionality using PathHunter β arrestin assay (DiscoverX). Follicle stimulating hormone acts through binding to its specific receptor. Binding of ligand to the receptor activates the adenosine 3',5'-cyclic monophosphate (cAMP) pathway (McFarland et al., 1989; Ji and Ji, 1991a; Rose, 1998) and the inositol 1phosphate (IP1) the second messenger systems. After stimulation of eelFSH receptor stably transfected Parental CHO cells with FSH wild type and mutant hormones as a ligand, production of cAMP and IP-1 were evaluated (Cisbio). cAMP IC-50 values by eelFSHwt; αΔ56; αΔ79; αΔ56_79; and βΔ5 were 33.1; 1154.7; 22; 410 and 311.9 ng/ml, respectively. IP-1 IC-50 values by eelFSHwt; αΔ56; αΔ79; αΔ56_79 and βΔ5 were 6.8; 7.1; 4.4; 3.8 and 10.2 ng/ml, respectively too. The cAMP activation was greatly decreased in the αΔ56αmutant. Thus, the site of α56 oligosaccharide in the eel plays an pivotal role for the cAMP stimulation using eel FSH receptor cell lines. In the IP-one assay, the activity in the αΔ56 and βΔ5 mutants was a little decreased than the wt. The biological roles of N-linked oligosaccharides in GPCR internalization are going to be estimated by measuring β arrestin recruitment system.