Italians are voting in a referendum which is being closely watched for further signs of anti-establishment sentiment in Europe.
The vote, called by centre-left Prime Minister Matteo Renzi, is formally on plans to streamline parliament but is expected to be used by many as a chance to register discontent.
Populist parties have campaigned for a No vote. Mr Renzi has said he will resign if he loses.
Opinion polls suggest he faces defeat.
Voting began on Sunday at 07:00 (06:00 GMT) and ends at 23:00, with results expected early on Monday.
In brief: the reforms include reducing the power of the senate. Its members would be cut from 315 to 100, with most drawn from mayors and regional representatives.
Mr Renzi says the reforms would speed up the cumbersome law-making process in Italy, which has had 60 governments since 1948.
Opponents say the proposals would concentrate too much power in the prime minister’s hands.
Some 50 million Italians have the right to vote in the referendum – many voters are fed up with years of economic stagnation.
An opinion poll in November gave the No vote a lead of at least five percentage points. But many Italians are thought to be still undecided.
The No campaign in Italy has been spearheaded by the the anti-establishment Five Star Movement, led by Beppe Grillo. It wants a referendum on whether Italy should keep the euro.
The Five Star Movement and the anti-immigrant Northern League would receive a boost from the prime minister’s defeat.
If Italy votes No, it would follow the UK’s vote in June to leave the European Union, as well as the rise of the anti-immigrant Front National in France and populist parties elsewhere (along with Donald Trump’s unexpected win in the US presidential election).
But the possibility of Mr Renzi, 41, falling from power has reignited concerns about financial stability in the euro zone’s third largest economy.
If Mr Renzi does lose, it is still not entirely certain that he will be out of power.
President Sergio Mattarella could ask him to form a new government or appoint a technocratic Prime Minister to serve until elections due in 2018.
“Do you approve the text of the Constitutional Law on ‘Provisions for exceeding the equal bicameralism, reducing the number of MPs, the containment of operating costs of the institutions, the suppression of the CNEL and the revision of Title V of Part II of the Constitution’ approved by Parliament and published in the Official Gazette n° 88 of 15 April 2016”?