3-Substituted 3,4-dihydrothieno[2,3-d]pyrimidin-4-one derivatives, production and use thereof. The invention relates to 3,4-dihydrothieno[2,3-d]pyrimidin-4-one derivatives which are substituted in position 3 by 5-membered heteroaryl which may be fused to an aryl or heteroaryl radical, where the heteroaryl and, if appropriate, the fused aryl or heteroaryl radical may have 1, 2 or 3 substituents selected independently of one another from C.sub.1-5-alkyl, C.sub.1-5-alkoxy, C.sub.1-5-alkylthio, halogen, CN, halo-C.sub.1-5-alkyl, halo-C.sub.1-5-alkoxy, hydroxy, --NH.sub.2, --N(R6).sub.2, --NH(R6), aryl, aryloxy, aralkyl, aralkyloxy and heteroaryl, where the substituents aryl, aryloxy, aralkyl, aralkyloxy and heteroaryl may have 1, 2 or 3 substituents selected independently of one another from C.sub.1-5-alkyl, C.sub.1-5-alkoxy, C.sub.1-5-alkylthio, halogen, CN, halo-C.sub.1-5-alkyl, halo-C.sub.1-5-alkoxy, hydroxy, --NH.sub.2, --N(R6).sub.2 and --NH(R6). The preparation and use of these derivatives, especially for therapeutic purposes, e.g. for the treatment of depressions, are likewise described.