A conclusive identification of the different conformers of the neurotransmitters ephedrine, norephedrine, and pseudoephedrine has been carried out in the gas-phase by molecular beam Fourier transform microwave (MB-FTMW) spectroscopy. Three conformers of norephedrine, three conformers of ephedrine, and four conformers of pseudoephedrine have been unequivocally assigned through the analysis of their rotational spectra and the comparison of the experimental rotational and N-14 quadrupole coupling constants with those calculated ab initio. The conformational preferences have been rationalized in terms of the various intramolecular forces at play. The main stabilizing interaction is an O-H center dot center dot center dot N hydrogen bond established in the side chain of the neurotransmitters which adopt an extended disposition in their most stable form.