N-Acetyl Semax (NA Semax, Ac-Semax) is the N-terminally acetylated form of Semax, a synthetic heptapeptide (Met-Glu-His-Phe-Pro-Gly-Pro) that joins the adrenocorticotropic hormone sequence 4-7 to a C-terminal Pro-Gly-Pro tripeptide. Acetylation of the free N-terminal amino group changes the chemical and biological properties of the parent peptide, and the stability of acetylated Semax to proteolysis in biological media has been examined. Published work on the acetylated form itself is limited to a small number of in vitro and physicochemical studies. Most of the literature concerns the parent compound Semax, studied in rat models of neurotrophin expression and focal cerebral ischaemia. Findings for Semax should not be assumed to apply to the acetylated derivative.