A method of preparation of (S)-N-methyl-3-(1-naphthyloxy)-3-(2-thienyl)propylamine of formula (I) or its pharmaceutically acceptable salt, in which (RS)-N,N-dimethyl-3-(1-naphthyloxy)-3-(2-thienyl)propylamine of formula (III) is reacted with an optically active acid, after which a crystallization is made of that diastereoisomer which yields, by reaction with an inorganic or organic base, (S)-N,N-dimethyl-3-(1-naphthyloxy)-3-(2-thienyl)propylamine of formula (S)-III, which is then demethylated using alkylchloroformates, followed by a hydrolysis and optional conversion of the compound of formula (I) to its salt.