Slovenian Prime Minister Janez Jansa called the change the "biggest national achievement" since the country joined the European Union two years ago, Ljubljana's media said.
Slovenia became the 13th EU member country to embrace the euro and the first of the 10 new EU members that joined the bloc in 2004.
Financial transactions, at the exchange rate of 239.64 tolars to one euro, will be carried out both in tolars and euros through Jan. 14.
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