Abstract: Cordioexeitatory peptide (Phe-Met-Arg-Phe-NH₂, FMRF-NH₂) and its analog (Phe-Pro-Arg-Phe-NH₂, FPRF-NH2) were synthesized by a solid phase procedure on a benzhydrylamine resin. Boo-amino acids and Boc-Arg(Tos) were used. The course of the synthesis was monitored by quantitative ninhydrin analysis. Following cleavage by the 90% HF method the peptides were purified by reverse phase HPLC. The overall yields of homogeneous isolated peptides from the first amino acid were 36% and 37%, respectively. It was found that the cleavage of Tos from Arg is related to temperature. The efficacy of FMRF-NH₂ and FPRF-NH₂ as antagonists to the opioid peptide D-Ala², Leu⁵ enkephalin was tested, using changes in mean arterial blood pressure and heart rate as indices. Results indicate that the former is more potent than the latter.