The present invention provides a process for the preparation of a tetrazine derivative of formula (I), or a pharmaceutically acceptable salt thereof wherein R1 represents a hydrogen atom, a straight or branched C1-C6 alkyl group, C2-C6 alkenyl group or C2-C6 alkynyl group, which C1-C6 alkyl group, C2-C6 alkenyl group and C2-C6 alkynyl group is unsubstituted or substituted with 1, 2 or 3 substituents selected from halogen atoms, straight or branched C1-C4 alkoxy groups, C1-C4 alkylthio groups, C1-C4 alkylsulphinyl groups, C1-C4 alkylsulphonyl groups and phenyl groups, which phenyl groups are unsubstituted or substituted with one or more substituents selected from C1-C4 alkyl groups, C1-C4 alkoxy groups and nitro groups or R1 represents a C3-C8 cycloalkyl group and R2 represents a group of formula —(C═O)NR3R4, wherein R3 and R4 are independently selected from hydrogen atoms, C1-C4 alkyl groups, C2-C4 alkenyl groups and C3-C8 cycloalkyl groups, which process comprises: i) providing a compound N of formula (III), wherein R1 is as defined R1—N═C═O ii) absorbing the compound of formula (III) into a solvent to obtain a solution of the compound of formula (III) iii) adding to the thus obtained solution a compound of formula (II), to obtain a compound of formula (I), as defined above, wherein R2 is as defined above iv) decomposing any excess compound of formula (III) remaining by addition of water and v) optionally salifying the thus obtained compound with a pharmaceutically acceptable acid, or base.
