The present invention refers to the chemical synthesis and potentiality for application in several biochemical assays, of a potent agonist of the .alpha.-melanocyte stimulating hormone (.alpha.-MSH), labeled with an amino acid-type paramagnetic spin probe (Toac, or 2,2,6,6-tetramethylpiperidine-1-oxyl-amino-4-carboxylic acid), valuable for use in electron spin resonance and fluorescence allowing biochemical-clinical investigation of relevant physiological roles of .alpha.-MSH in animal organism. The referred analogue of the present invention, acetil-Toac.sup.0-(Nle.sup.4, DPhe.sup.7)-.alpha.-MSH, where (Nle.sup.4, DPhe.sup.7)-.alpha.-MSH is nominated commercially as Melanotan.TM. and although containing a non-natural compound in its structure such as Toac, maintained entirely its potent agonist potency. Its integral activity associated with the fact that it is naturally fluorescent (due to the Trp residue of its sequence) give to this labeled analogue a unique potential for a great variety of investigations regarding relevant physiological roles already known of this neuroimmuno-modulator.