The emergency measure meant to protect shares of 19 financial companies from abusive short-selling is set to expire tomorrow. The SEC has said that it will not extend the rule. Instead, it plans to propose a new rules on short-selling that will apply across all US equities markets. But that rule could be months in the offing.

It's not clear what effect the emergency rule had. Some market participants say the requirement that short sellers manually locate and pre-borrow shares slowed down short trading. Others say that shares in every one of the 19 protected companies--including mortgage giants Fannie Mae and Freddie Mac and Lehman Brothers--were widely available for borrowing, and that any slow down was minimal.

The new rules could take a variety of forms. A reintroduction of the old uptick rule, which required short sellers to wait for stocks to move higher before they added a short position, is unlikely. But some say a modified uptick rule, essentially a circuit breaker that would prohibit shorts if a stock was rapidly declining, may be introduced. Other possibilities include mandatory disclosure of short positions and the application of some version of the emergency rule to all shares traded on US exchanges.

One dark-horse possibility emerged recently in Pakistan, where some investors recently urged that a ban on all stock declines be adopted. Freddie Mac chief executive Richard Syron could not be reached for comment.