Tehran's favorite parlor game these days is doping out what happens when, rather than if, the once unthinkable departure of Shah Mohammad Reza Pahlavi actually happens.

Seemingly endless permutations of speculation abound, involving regency councils, Latin American-type plots and coups in the armed forces, or well meaning but ineffective Kerensky-like civilian government. Few options are optimistic.

That may prove to be the shah's insurance of remaining on the trone he's occupied for the past 37 years, although no one can be found who thinks he'll survive as more than a much chastened constitutional monarch.

Shorn of his once absolute powers, the shah could be forced to abdicate in favor of his 18-year-old son, Crown Prince Reza.

In that case, an eight-member regency council -- the prime minister, the presidents of both upper and lower houses of parliament, the senior appeals court judge and four government appointees -- would preside over the realm until Reza reaches his constitutional majority at age 21.

The advantages of such a system are that the Pahlavi dynasty continues, a break in tradition is averted and hopefully a modicum of stability and continuity is maintained.

The regency council appeals to the so-called constitutionalist opposition which may hope privately to end the monarchy but prefers a step-by-step approach to the uncertainty of full-fledged revolution.

Some shah loyalists insist that his abdication would mean the dynasty would be swept away in the ensuing torrent. While they might credit the crown prince as being a pleasant, normal young man, they say he is scarely equipped to handle such a crisis.

More serious objections are voiced by the shah's arch-rival, Paris-based exile Ayatollah Ruhollah Khomeini. He wants to end the monarchy and set up a yet ill-defined Islamic republic.

Other objections are reported from the armed forces, whose leaders reportedly refuse their crucial support for any solution calling for the present shah's ouster.

That would all but rule out a civilian government. Even if it did manage to take office it would be short-lived if this scenarlo is to be believed.

Another major option involves the military forcing the shah out despite their loyalty, previously taken for granted.

At least two serious variances exist. Both assume than an armed forces move would be based on the conviction that the shah has become a liability rather than a force for unity and stability.

The first variant involves factions and coups in the army as the various opposition forces -- Islamic, Marxists, nationalists, liberals and others -- try to line up support inside the military.

This is the so-called South American approach, notable for its penchant for revolving-door coups.

Yet another subrefinement exists because of the influence of Shiah Moslem leaders in the present crisis.

The coup leaders could invoke Islam with the main question being whether their model would be the Islamic military government Shite Pakistan or that in Libya.

The second major military variant is that old Third World favorite -- the man on horseback, probably a younger officer with reform in his political program if not on his mind, the Iranian version of the late Gamal Abdel Nasser's much-copied free officers movement in Egypt.

The only possibility universally dismissed by game players is Khomeini's Islamic Republic. The constitutionalist opposition leaders long felt threatened by Khomeini although they now say he has assured them that he has no intention of running the government himself.

Accepted even by the shrinking number of specialists who bet on the shah's remaining in power is the coming of a major shift of Iranian politics.

Not even optimists believe Iran's domestic problems will prove easy to solve, no matter who is trying to rule.

Already Iran has lost kind of influence aboard which allowed the shah to lecture the West on its decadent ways, seize Persian Gulf islands, and send troops to put down a Marist-backed rebellion in Oman.

No longer likely to acquire modern arms on such a massive basis, Iran is also less likely to scare its neighbors, especially the less populated and weaker Arab nations.

Perhaps overly optimistically, a Western ambassador suggested recently that no matter who eventually took power, Iranian self-interest would dictate continued oil sales to the West.

"We're not worried about another Afghan situation," he said, referring to the Marxists who seized power in Iran's eastern neighbor last April.

Yet, growing criticism of the West already has proven a favorite tactic for Iranians of all political persuasions -- and not just opposition firebrands -- eager to transfer responsibility for the country's problems.