Russia has ordered five Moldovan diplomats to leave the country, a statement says, two days after Moldova expelled five Russian diplomats.
The Russian foreign minister criticised the "counter-productive" nature of Moldova's "unfriendly actions".
The diplomats, who were not named, were given three days to leave Russia.
Moldova's pro-EU government has not explained the expulsions. But the prime minister said there were "good reasons" for it.
"It was a matter of the country's national interests and the state security of Moldova," Pavel Filip said on Wednesday in a statement quoted by Reuters news agency.
"We are in favour of good relations with Russia, but this relationship should be built of mutual respect."
The country's President Igor Dodon, who sees close ties with Moscow as a priority, had earlier called the move against the Russian delegation "outrageous".
Moldova is one of Europe's poorest countries and gets funding from the EU.
In March a former Moldovan MP, Yuri Bolbochan, was arrested and accused of treason for allegedly having passed secrets to an aide to the Russian military attaché.
The aide - named by Russian media as Alexander Grudin - is reported to be among the five diplomats ordered to leave Moldova.
Moldova signed a far-reaching association agreement with the EU in 2014, and Russia promptly imposed import restrictions on the country's agricultural produce.
Russian troops are stationed in the breakaway Trans-Dniester region, which is economically dependent on Russia and defies the Moldovan government in Chisinau.
In 2015 Moldova was rocked by a huge fraud case, when more than €1bn (£867m; $1.1bn) disappeared from three banks.
The conflict in neighbouring Ukraine has raised tensions in Moldova, an ex-Soviet republic that Russia sees as part of its sphere of influence.