Speaking with rural leaders in Saskatoon Thursday, Harper said that retaliation will become essential at some point in the dispute.
"We are prepared to retaliate at a point in time," the prime minister told the audience. "Obviously we don't want to retaliate in a way that would endanger our wider trade interests, but I will say that at some, point retaliation becomes essential."
Harper did not take question from reporters.
American country-of-origin rules require all packaged meat to identify where the animal was born, raised and slaughtered.
Supporters of the law say it better informs U.S. consumers, but opponents argue that segregating animals and tracking them adds costs.
Canada and Mexico say the U.S. rules violate international trade law and have damaged their industries, which is why the two countries are fighting the rules at the World Trade Organization.
Late last year, the United States filed its final appeal of a WTO ruling that found its meat labelling laws discriminate against Canadian beef and pork exports.
Canada has said it is poised to slap retaliatory tariffs on a range of U.S. products, including pork, beef, wine and orange juice, if it wins the next round at the WTO and the U.S. doesn't adjust its rules.
Last month, however, Federal Agriculture Minister Gerry Ritz said he was hopeful things wouldn't get that far, based on what he's heard from lawmakers in Washington.