As the crisis worsens for mortgage giants Fannie Mae and Freddie Mac , Treasury Secretary Henry Paulson is insisting that any potential government rescue plan not benefit the companies' shareholders, according to people familiar with the matter.

The two stockholder-owned, government-sponsored companies, whose operations are vital to the functioning of the U.S. housing market, faced a severe crisis of confidence after a week in which their stocks each lost nearly half their value. On Friday, Freddie Mac finished the day at $7.75 a share, and Fannie Mae at $10.25.

The discussions at Treasury highlight the dilemma created by the financial crisis gripping the U.S: Some institutions are considered too big to fail, but propping them up could erode the market's incentive to properly judge risk by offering investors a false sense of security.

After a week of near panic among shareholders of the two companies -- and a stomach-churning day on Wall Street Friday -- the next big test will come Monday when Freddie Mac is due to sell $3 billion of short-term debt. An unsuccessful sale could be a major blow to investor confidence. If the administration were to intervene, it could do so before markets opened that day, according to a person familiar with the deliberations.