REPRESENTATIVES of three states and the District of Columbia plus the Reagan administration's top environmental official gathered in Baltimore Tuesday to sign a most important pact -- pledging their governments' best efforts to save the gasping Chesapeake Bay and bring back the natural life and wonder of this country's largest estuary. As Virginia Gov. Gerald L. Baliles said on the eve of the signing, "It is difficult to overstate the importance of this new pact" -- a document that sets specific goals and timetables for reviving the bay. Joining Mr. Baliles as signers were Maryland Gov. William Donald Schaefer, Mayor Marion Barry, Pennsylvania Gov. Robert P. Casey, Environmental Protection Agency administrator Lee Thomas and Kenneth Cole, a Pennsylvania legislator and chairman of the Chesapeake Bay Commission.

Serious effort went into this agreement, and though it sidesteps the issue of immediate, direct action against toxic pollution from industrial plants, the pact does commit all parties to come up with a plan to alleviate this pollution and to act within a year. The document also sets a target of the year 2000 for reduction by 40 percent of the nitrogen and phosphorous from sewage plants and farm run-off that has been killing aquatic life -- consuming the oxygen and choking the underwater grasses. Other commitments are to reduce pollution from federal facilities; to remove dams or provide passages around them as well as other stream blockages, which would allow migratory fish better passage into the tributaries; and to protect stocks of blue crabs, oysters and shad.

Though these and other provisions amount to the most comprehensive effort yet to rescue the bay, a pact is nothing more than what happens after it's signed. Results will require not only enormous appropriations but also victories in what will surely be bitter battles over private property rights, industrial prerogatives and the lifeblood of the watermen. Still, the fact that government leaders have recognized the requirement for a coordinated regional approach can be important in achieving results. As former Maryland senator Charles McC. Mathias Jr., a pioneer in this effort, noted, "From now on, the bay will be viewed as the crabs and fish see it -- without political boundaries, without artificial borders."