The article analyses the reactivity level of aggregates from Lithuanian quarries and their effect on the alkali corrosion in mortars. The reactivity of aggregates was assessed according to Rilem Recommended Test Method: AAR-2. The hardened cement paste and cracked aggregate contact zones were tested by means of optical microscopy. Two gravel aggregates and one sand aggregate from Lithuanian quarries were selected for testing. The tests revealed that according to AAR 2 methodology fine and coarse aggregates from Lithuanian quarries shall be allocated to Group II, i.e. reactive aggregates. The expansion after 14 days exceeds 0.1 % in the case of fine aggregate, the average expansion after 14 days is 0.13 %, and in the case of coarse aggregates the average expansion ranges from 0.11 % to 0.12 %.