The nation's economy grew at an annual rate of only 0.7 percent during the fourth quarter of last year -- so slowly that Congress may be forced to vote on suspending the Gramm-Rud-man-Hollings budget-cutting law for the new government fiscal year that begins in October.

Unless the economy picks up, the slow growth will trigger a provision of the law designed to make it possible to override the law's budget-deficit ceiling when the economy slumps.

The Commerce Department yesterday lowered its estimate of economic growth in the last three months of 1985 from 1.2 percent a year to 0.7 percent. Gramm-Rudman-Hollings requires the government to reconsider its mandatory budget cuts if the economy grows at less than a 1 percent rate for two quarters in a row.

Most economic forecasters expect the first quarter economic growth to be better than 1 percent when the figure is reported next month, but many uncertainties in the economy could produce slower growth.

Asked about the likelihood of having a second such weak quarter in a row, economist Alan Greenspan of Townsend-Greenspan & Co. replied, "That probability is not zero."

Greenspan still expects first-quarter growth to be higher than 1 percent, but some recent developments, such as major cutbacks in spending by oil companies on exploration and development in the wake of the big drop in oil prices, have made him less certain.

Moreover, some of the changes in the fourth-quarter estimate also may limit the first-quarter gain, Greenspan said.

House Budget Committee Chairman William H. Gray III (D-Pa.) said it is too early to consider suspending Gramm-Rudman-Hollings. "I would want to see what the first quarter would be before I would want to talk about lifting Gramm-Rudman," Gray said.

He said he expects real output to rise at a 3 percent pace or higher. If it turns out to be much lower, he also would want to know how long growth is expected to remain low before deciding whether suspending the deficit target would be appropriate, he added.

The 0.7 percent figure for the fourth quarter was the smallest for a quarter since the 0.6 percent rate of increase in the final three months of 1984. The downward revision left real GNP up 2.2 percent in 1985 from 1984. That was the smallest annual rise in the output of goods and services since a 2.5 percent decline in the recession year of 1982.

An extended period of slow economic growth can reduce federal revenue below expected levels and therefore could make inappropriate a deficit target set in advance, such as those in Gramm-Rudman-Holl-ings. The requirement for formal consideration of suspension of the target and the process of automatic spending cuts was added to the final version of the law for that reason.

The spending cuts already put into effect for the current fiscal year as a result of the new law would not be affected.

Under Gramm-Rudman-Hollings, two quarters of slow growth require that a joint resolution be introduced to suspend the following fiscal year's deficit target. If the resolution passes, the president can sign or veto it.

Virtually all of the fourth-quarter downward revision in the gross national product, adjusted for inflation, was the result of estimates that the nation's trade deficit was running at an annual rate about $6.5 billion worse than had been thought.

That change more than offset smaller upward revisions in estimates for business investment in plants and equipment and in inventories. Government purchases of goods and services also were revised upward somewhat, while residential investment went down slightly compared with the earlier figures.

In December, before the fourth quarter ended, the Commerce Department's bureau of economic analysis reported in its so-called flash estimate that real GNP was rising at a 3.2 percent annual rate. The preliminary estimate in January pegged growth at 2.4 percent, which then was lowered to 1.2 percent in the first regular revision released last month. Yesterday's 0.7 percent figure was the second regular revision.

Commerce announced earlier this year that it no longer would make flash estimates because of their unreliability. Otherwise, such a flash estimate for the first quarter would have been released yesterday.

The estimate for inflation for the quarter, as measured by the GNP price index, was left unchanged at 3.9 percent in the latest revisions