Twenty-four thyrotropin-releasinghormone (TRH) analogues containing mainly aliphatic amino acids in position 2 were synthesized and tested for central nervous system (CNS) and hormonal (TSH) activity. Application of the pentafluorophenyl ester method in the syntheses resulted in optimal yields and high purity of the products. The neutral tripeptides pGlu- Nva -Pro-NH2 (9), pGlu-Nle-Pro-NH2 (10), and