The departure of five health plans from the federal insurance program could cause major problems for workers, retirees and dependents who routinely run up heavy bills for treatment of mental illness or substance abuse.

Most of the traditional fee-for-service plans have lifetime mental health limits ranging from $40,000 to $50,000. Because people can switch plans every 12 months, many simply move into a new plan when they approach or use up their current plan's lifetime limit. The new plan must take them, and they start from zero with its lifetime benefit limit.

Some people switch plans every year. The Blue Cross high option and the Foreign Service health plan each have a $75,000 lifetime mental health benefit limit. The Mail Handlers and American Postal Workers Union plans have each the lowest ($25,000) lifetime mental benefit limit. Check brochures carefully, because some plans make a distinction between mental health and substance abuse benefits and limits.

In 1990, the typical federal worker or retiree could pick from about 20 fee-for-service or local health maintenance organizations. Next year there will be five fewer. Dropping out are the American Federation of Government Employees, National Federation of Federal Employees, National Association of Government Employees and National Association of Postal Supervisors plans. Also leaving the program is GEBA, which is not to be confused with the popular GEHA plan. GEBA is going; GEHA is staying.

Most workers and retirees currently enrolled in the plans that are dropping out will be able to switch to another plan during the open season -- which ends Monday -- and get equal or better coverage at less cost. But for individuals or families who routinely have big mental health bills, the dwindling number of plans is yet another thing to worry about. Political Funds

Active and retired members of the National Association of Letter Carriers donated $2.6 million to the union's political education committee during the 1989-90 election cycle.

Because union dues cannot be used for political purposes, many federal, postal and retiree groups have their own political action committees funded by separate contributions from members.

The letter carriers and the National Association of Retired Federal Employees have two of the biggest PACs in the nation. Pay Differentials

Beginning in 1992, federal law enforcement officers in a dozen cities will be paid cost of living differentials.

A 16 percent differential is slated for the Boston-Lawrence- Salem metropolitan area, Los Angeles- Anaheim-Riverside areas, New York-Northern New Jersey- Long Island areas, and San Francisco-Oakland-San Jose areas.

Differentials of 8 percent will be paid in the San Diego metropolitan area.

Cities slated for a 4 percent differential include metropolitan Chicago, Philadelphia-Trenton- Wilmington and the Washington area.