The tetahydroimidazopyridine (I) is prepared by condensation of histamine with propionaldehyde in the presence of NaOH. The alkylation in position 1 or 3 is performed with ethyl bromide in a catalytic two-phase system, after protection of the amine group in position 5 as benzyloxycarbonyl derivative, and followed by removal of the protecting group by catalytic hydrogenation. The resulting amines (IVa-b) are then made to react with isopropyl isothiocyanate and the two regioisomers obtained are separated by crystallization.