TORONTO (AP) _ Prime Minister-designate Jean Chretien today reiterated his opposition to the North American Free Trade Agreement and said he may refuse to put Canada's ratification of the treaty into formal effect.

Chretien, whose Liberal Party won a landslide victory in Monday's national elections, has pledged to renegotiate NAFTA with the United States and Mexico.

At his first formal press conference since the election, he noted that under the Canadian system a law does not go into effect until it is officially proclaimed. That has not yet been done with the ratification legislation passed by Parliament in May, and Chretien said he might refuse to proclaim it.

But the 59-year-old Quebec lawyer declined to say if he was seeking concessions from Washington before proclaiming it.

Chretien reiterated his party's position on the original Free Trade Agreement with the United States, which went into effect Jan. 1, 1989, and the NAFTA pact scheduled to go into effect Jan. 1, 1994, if passed by the U.S. Congress.

''In our program, we were asking for changes to NAFTA,'' he said. ''Two have been dealt with, labor and environment.'' He said three more are left - exact definitions of subsidies and dumping and changes in the provisions concerning energy.

The original Free Trade Agreement, which would be superseded by NAFTA, obligated the United States to negotiate a code with Canada on dumping and subsidies within the first seven years, Chretien said.

Chretien said there was no substantive discussion on NAFTA when he spoke with President Clinton on Tuesday.

''They know my position,'' he said.

Chretien said his first priority as prime minister would be ''to work on the economy, the reason we won the election.''