The 92-year-old former president known for his post- presidential humanitarian work was taken to a hospital in Winnipeg as a precaution for rehydration, spokeswoman Deanna Congileo said.
Habitat for Humanity CEO Jonathan Reckford said the former president did not appear to be in serious medical danger.
"President Carter told us he is OK and is being taken offsite for observation," Reckford said. "He encourages everyone to stay hydrated and keep building."
Carter had joined Habitat as the organisation builds 150 homes for people in need in Canada to celebrate the country's 150th anniversary.
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Photos showed the former president in jeans, long-sleeved shirt and red hard hat using a hand drill at the work site.
Sara said he was scheduled to return tomorrow but it was not clear whether he would.
Carter had been working for about 90 minutes when he went to sit down in a chair, the Winnipeg Free Press reported. He appeared to wobble, so Secret Service agents led him to his motorcade. He was treated by paramedics who took him away in an ambulance, without using lights or siren, the newspaper said.
Carter was diagnosed with melanoma that spread to his brain in 2015 but announced in March of the following year that he no longer needed treatment. Carter continued to volunteer for Habitat while being treated for cancer, working alongside volunteers at a home in Memphis in November 2015.