In vivo bioconjugation to the free thiol on Cys34 of serum albumin by a strategically placed reactive group on a bioactive peptide is a useful tool to extend plasma half-life. Three maleimido derivates of human GRF_1-29 were synthesized and bioconjugated to human serum albumin (HSA) ex vivo. All three HSA conjugates showed enhanced in vitro stability against dipeptidylpeptidase-IV (DPP-IV) and were bioactive in a growth hormone secretion assay in cultured rat anterior pituitary cells. When the maleimido derivatives were individually administered subcutaneously to normal male Sprague Dawley rats, an acute secretion of growth hormone (GH) was measured in plasma. The best compound, CJC-1295 showed a 3-fold increase in GH AUC over a 2-hour period compared with human GRF_1-29. CJC-1295, a tetrasubstituted form of human GRF_1-29 with an added N-3-maleimidopropionamide derivative of lysine at the C-terminus, was selected for further pharmacokinetic evaluation, where it was found to be present in plasma beyond 72 h. A Western blot analysis of the plasma of a rat injected with CJC-1295 showed the presence of a CJC-1295 immunoreactive species on the band corresponding to serum albumin appearing after 15 min and remaining in circulation beyond 24 h. These results led to the identification of CJC-1295 as a stable and active hGRF_1-29 analog with an extended plasma half-life.