Russia has ordered three Lithuanian diplomats out of the country, one month after Lithuania itself expelled three Russian diplomats for allegedly spying.
Lithuania expelled three Russians in February
Russia's Foreign Ministry said it had expelled the diplomats for "activities incompatible with their status", a phrase often used to imply espionage.
They have been given two days to leave the country.
Russia had warned Lithuania it saw the earlier expulsions as an "unfriendly act" which would provoke a response.
Lithuania expelled the three Russian diplomats in February, accusing them of spying and trying to gain access to Nato and European Union secrets.
It also said the diplomats had been illegally trying to influence privatisation and obtain information from parliament about allegations against Lithuanian President Rolandas Paksas.
Mr Paksas is facing impeachment proceedings in parliament after his office was accused of having links to Russian crime gangs.
A vote on the proceedings is expected in April.
Lithuania's constitutional court has also opened an inquiry to decide whether he violated the constitution.
Mr Paksas says he is innocent and has refused repeated calls for him to resign.
Former Soviet republic Lithuania, which joined Nato on Monday along with six other eastern and central European states, has uneasy relations with Russia, which has become increasingly nervous of Nato's eastward expansion.
Nato is planning to deploy aircraft to defend the airspace of Lithuania, along with Latvia and Estonia, as soon as they join the alliance.
Russia's Defence Minister Sergei Ivanov had warned such plans would lead Moscow to reassess its military planning, including a possible build-up of the country's nuclear defences.