The convertible currency on the territory of the Russian Federation is ruble.
The contemporary Russian ruble was introduced in December 1991 together with the Soviet ruble. Their common usage continued in the RF until September 1993. All the Soviet coins, produced in 1961-1991 formally stayed legal payment means until December 31, 1998, and in 1999-2001 they could be exchanged to the Russian currency at the rate of 1000:1.
Today in Saint-Petersburg it is not a problem to exchange the international currency into the Russian rubles. It is better to do that in banks or the special exchange booths. Never exchange money in private persons. You will most likely lose both foreign currency and rubles.
1 EUR = 90.4602 RUR
1 USD = 75.5535 RUR