The Canadian dollar slipped below 91 cents US on Tuesday to 90.91 ahead of an interest rate announcement set for Wednesday from the Bank of Canada.
It had recovered slightly by noon, rising to 91.09 US.
The S&P/TSX composite index dropped 21 points to 13,968.
The Bank of Canada is not expected to change its key overnight lending rate on Wednesday, though there is pressure from some to lower rates to keep the dollar higher.
Canada's central bank hasn't moved its benchmark lending rate since September 2010.
Bank of Canada governor Stephen Poloz has said he is worried about the slow recovery of Canadian exports and he may prefer to leave the interest rate low to make it easier for Canadian manufacturers and producers to sell their goods.
RBC allowed its fixed-rate mortgage interest rates to fall by 10 basis points on Monday, reflecting lower yields on Canadian bonds.
The loonie is under pressure because traders are moving back to the U.S. dollar amid signs of a recovering U.S. economy.
The Canadian dollar is trading at its lowest levels since 2009, after falling from par in February 2013.