Abstract: The present invention provides a process for the preparation of 5,6,7,7a-tetrahydro-4H-pyridin-2-one derivatives of the general formula: ##STR1## in which R is a hydrogen atom or a phenyl radical optionally substituted by at least one halogen atom, lower alkyl radical, lower alkoxy radical, nitro group, carboxy group, alkoxycarbonyl radical or cyano group; R' is a hydrogen atom or a lower alkyl radical and n is 0, 1, 2, 3 or 4; and of their addition salts with mineral or organic acids, wherein a compound of the general formula: ##STR2## in which R, R' and n have the same meanings as above and R" is a hydrogen atom or an alkyl radical containing up to 4 carbon atoms, is treated in an organic solvent with gaseous hydrogen choloride and gaseous hydrogen sulphide. The compounds have platelet anti-aggregant and anti-thrombotic properties.