Substituted 3-phenyl-5-alkoxy-1,3,4-oxadiazol-2-ones of the formula 1 are described, in which R1 is substituted C1-C6-alkyl and C3-C9-cycloalkyl, R2, R3, R4 and R5 are hydrogen, halogen, nitro, C1-C4-alkyl, C1-C9-alkyloxy, substituted C6-C10-aryl-C1-C4-alkyloxy, C6-C10-aryloxy, C6-C10-aryl, C3-C8-cycloalkyl or O—C3—C8-cycloalkyl, or 2-oxopyrrolidin-1-yl, 2,5-dimethylpyrrol-1-yl or NR6-A-R7, with the proviso that R2, R3, R4 and R5 are not simultaneously hydrogen, and at least one of the radicals R2, R3, R4 or R5 is the radical 2-oxopyrrolidin-1-yl, 2,5-dimethylpyrrol-1-yl or NR6-A-R7, wherein R6=hydrogen, C1-C4-alkyl or substituted C6-C10-aryl-C1-C4-alkyl, A=a single bond, COn, SOn or CONH, n=1 or 2, R7=hydrogen, substituted C1-C18-alkyl, C2-C18-alkenyl, C6-C10-aryl-C1-C4-alkyl, C5-C8-cycloalkyl-C1-C4-alkyl, C5-C8-cycloalkyl, C6-C10-aryl-C2-C6-alkenyl, C6-C10-aryl, biphenylyl, biphenylyl-C1-C4-alkyl, indanyl, or the group Het-(CH2)r—, wherein r=0, 1, 2 or 3 and Het=a saturated or unsaturated 5-7-membered heterocycle, which may be optionally benzo-fused and optionally substituted, and proceses for their preparation. The compounds of formula 1 show an inhibitory effect on hormone-sensitive lipase, HSL.