The Czech Pharmacopoeia 1997 takes over estimation of the stomatal index (S.I.) from the 3 rd edition of the European Pharmacopoeia and lists it in the pharmacognostic methods of drug testing. In the leaves of selected representatives of the Mentha L., values of S.I. were examined in various parts of the blade and an investigation was made to find to what extent this value is stable and usable for distinguishing the individual representatives. In all samples there were cases where the S.I. values found in one leaf significantly differed. The highest probability of distinguishing samples is on the basis of the values found in the upper margin and the middle centre of the blade. When comparing the individual samples on the basis of the average S.I. calculated from the values measured in all six parts of the blade, it is possible to distinguish all samples, except the combinations M. piperita „Prilubskaja – M. aquatica var. bergamotta, M. piperita „Kliment“ – M. piperita var. vulgaris „Mitcham“, and M. piperita „Kliment“ – M. japonica, with the use of I.S.

        Key words: quantitative microscopy – stomatal index – Mentha L.