The medicinal plants belong to main natural sources of therapeutical substances. In the past these plants have been obtained by individual collection in the particular localities of their natural growth, which is at recent conditions of quantitative and qualitative development of the pharmaceutical industry not sufficient. A starting point from that situation lies only in the planed and organized cultivation of the medicinal plants. In order to increase their yields it is necessary to select a suitable cultivar, locality, technique of growing along with appropriate selection of the plant protection formulations.

In the frame of long-term task involving the study of effect of various unwanted substances on the drug quality, we have focused the attention to application of the Hattrick preparation, which is used in the Mentha piperita Huds. croppings against the Puccinia menthae rust. The goal of work was to evaluate the content and composition of the essential oil from peppermint during its vegetation periods in individual plant parts after the application of protective preparation. Listed parameters were studied in the main development phases, i.e. beginning of the stalk formation, beginning of flowering, full flowering, and the phase of biological ripe. The phase exerting the highest quality and essential oil content is that of beginning of flowering. The same was observed for both untreated (control) and treated plants. At the experiments during the vegetation we have found out that the application of given fungicide did not affect (speed up) the process of beginning of flowering, neither the phase of full flowering, which are important for the peppermint harvest.

From the results and corresponding values is evident that the beginning of flowering may be regarded as the phase yielding the highest content of the essential oil. Moreover, the highest essential oil content was found in the top, developmentally youngest group of leaves. These findings were not affected by the application of Hattrick, neither the remarkable changes in the relative abundance of individual oil components under study were detected. Thus, the content of most significant component menthol was gradually increasing during the plant vegetation and reached its maximum at the beginning of flowering.