Mentha × piperita (L.) Huds. of the family Lamiaceae is a very important species for commercial exploitation due to the high content of essential oil. Besides the essential oils, there are also other significant secondary metabolites in peppermint, especially flavonoids and hydroxycinnamic derivatives. In this study, we evaluate the variability of essential oil constituents and of phenolic compounds in the leaves of M. × piperita during the development of inflorescence. Similar studies, dealing with the variation of the essential oils and its composition, have been performed in the past, but no attention was paid to the variation of other secondary metabolites. We examined M. × piperita cv. ‘Perpeta’, the domestic cultivar. The plants were cultivated in the climatic conditions of south-west Slovakia. The contents of secondary metabolites were investigated in the week periods, from the beginning of inflorescence formation to the end of flowering. The yield and the quality of essential oil have been carried out by distillation and gas chromatography mass spectrometry (GC-MS). The phenolic substances were analysed using spectrophotometric methods according to European Pharmacopoeia. The highest contents of phenolic substances have been found in leaves of plants during the flowering phenophase, the same stage when essential oil of mint also achieves the highest quality.