Three mafic dykes found in the vicinity of Blansko were examined petrographically. One of them was studied geochemically. All three dykes contain clinopyroxene as phenocrysts (sometimes pseudomorphosed by hornblende) or in the groundmass with lathy plagioclases in the central parts of the dykes. Plagioclase usually takes the form of euhedral phenocrysts toward the contact with the host rock. Also, the structure changes from ophitic in the central parts to porphyric along the contact. Studied dykes display modal composition of microdiorites and porphyric microdiorites respectively. The dyke from Čertův Hrádek (no. 1) shows a noticeable geochemical similarity compared with the Hercynian dykes of porphyric microdiorites (diorite porphyrites) of the eastern granodiorite part of the Brno Massif studied by Hanžl et al. (1995). The dyke is formed by basaltic andesite based on the alkali/SiO2 ratio with features of calc-alkaline within plate basalt due to content of some trace elements or non-orogenic basalt account on content of Ti in pyroxenes (Leterrier et al. 1982).