Lithuania has started shutting down one of the reactors at its only nuclear power station, in line with European Union entry conditions.
Ignalina started generating power in 1983
Unit One at the Soviet-era Ignalina plant, north-east of Vilnius, is to stop functioning before midnight.
It is similar to the Chernobyl reactor which blew up in 1986 in Ukraine.
The Ignalina plant - supplying about 70% of the Baltic state's energy - has two RBMK reactors, with a capacity of 1,300 megawatts each.
Lithuania, which joined the EU in May, pledged to close the entire facility by the end of 2009.
The EU has been worried about safety at the plant, which lies near the town of Visaginas. It has pledged almost two billion euros (£1.3bn) to help Lithuania close the plant.
The plant's director, Viktoras Sevaldinas, said the Chernobyl disaster had "cast a shadow over our plant, too".
"We knew the first unit would be closed long ago and we prepared for it."
The Associated Press reports that both reactors at Ignalina underwent safety upgrades after Lithuania regained its independence from the Soviet Union in 1991.
"Only after five years we will be able to remove nuclear fuel containers from the first unit. Later we will start to dismantle the reactor. The complete process could take as long as 30 years," Mr Sevaldinas said.