Human chorionic gonadotropin (hCG), luteinizing hormone, follicle-stimulating hormone (FSH), and thyrotropin (TSH) are hormones that share a common alpha subunit but differ in their beta subunits. Recombinant DNA techniques, valuable tools for structure-function analyses, provide an approach for designing therapeutic analogs. FSH is used clinically to stimulate the ovarian follicles for in vitro fertilization and to initiate follicular maturation in women with infertility problems. The CG beta subunit contains a carboxy-terminal extension (CTP) with four serine O-linked oligosaccharides, which is important for the long half-life of hCG. A clinical problem of FSH is its relatively short half-life in circulation. Fusing CTP to the FSH beta coding sequence increased the in vivo potency of the resulting FSH dimer over three-fold. Analogs of the other hormones containing CTP also increase their biologic half-life. Subunit assembly is vital to the function of these hormones. To address whether alpha and beta subunits can be synthesized as one chain and also maintain biological activity, a chimera comprised of the hCG beta subunit genetically fused to the alpha subunit was constructed. The resulting polypeptide was efficiently secreted and displayed an increased biologic activity in vitro and in vivo. Similarly, the single-chain form of FSH also retained in vivo activity. Since subunit dissociation inactivates the activity of the heterodimer, single-chain analogs should have longer biological half-lives. These analogs represent suitable substrates for engineering potent and stable agonists and antagonists.