The Thornhill riding — held previously by Progressive Conservative Peter Shurman — is billed a two-way contest between the governing Liberals and the PCs.
In Niagara Falls the New Democrats and Progressive Conservatives are in a close race despite the fact the riding has been held for 10 years by Liberal Kim Craitor, who resigned in September.
Premier Kathleen Wynne said people know they can vote against the Liberals in the byelections without changing the status of the minority government.
Wynne also said the outcomes won't alter her plans to introduce a budget, and rejects suggestions she'd want to call a spring election if the Liberals manage to take both ridings.
PC Leader Tim Hudak is under pressure to hold Thornhill, but if the Tories can also take Niagara Falls party insiders expect the NDP will quickly strike a budget deal with the Liberals to avoid a spring election.
However, if the Conservatives should lose both byelections, there will be extra pressure on NDP Leader Andrea Horwath to stop propping up the minority government and trigger an election. A poor showing by the Tories in both races would also put pressure on Hudak, who has faced recent caucus unrest.
More than 10,000 people cast votes in advance polls in the two ridings. Polls close 9 p.m.