The Justice Department yesterday dismissed two antitrust suits filed against businesses by the Carter administration, providing new indications of a major shift in national antitrust enforcement policy. The action marks the first time since 1976 that the department has dismissed a case after it was filed, a spokesman said.

Dismissal of the two suits by William Baxter, assistant attorney general for antitrust, could add impetus to a growing merger trend among the nation's large companies, highlighted earlier this week when E. I. du Pont de Nemours announced plans to buy Conoco Inc. in a combination that would create the nation's seventh-largest industrial corporation.

The decisions announced yesterday, involving alleged price-fixing and regional business domination by a few firms, are the first products of a major review of at least six pending antitrust actions undertaken by Baxter, an appointee of President Reagan and a former law professor from Stanford University.

Baster has questioned many of the premises of his predecessors and has brought about a legal climate that lawyers see as more favorable to corporate mergers. Baxter also has said he favors eliminating the antitrust mission of the Federal Trade Commission.

Both dismissals followed requests to Baxter to review the cases by lawyers representing the two companies. "The decision was made just as the cases were brought -- soley on the merits," said a department spokesman. "The people who brought the cases probably still believe they should be brought to trial."

One of the cases dismissed yesterday was brought against Mack Trucks Inc. and the National Distributor Council Advisory Council, a group of independent distributors who are appointed by the trucking company to sell Mack products. The suit was brought on Jan. 12, just eight days before the close of the Carter administration. It charged the trucking concerns with violating the Sherman Antitrust Act by agreeing to pricing discounts for Mack truck parts.

In effect, the Mack decision means that Baxter is likely to view efforts by suppliers to set resale prices for their goods as legal, a reversal of policy since the Carter administration.

"Vertical arrangements [companies controlling manufacture, distribution and sales of products], in and of themselves, do not impair the efficiency of markets or lessen the welfare of consumers," Baxter said of the Mack case. "They may do so, depending on the structure of the industry, if they facilitate . . . collusion either at the manufacturing level or at the distribution. But it did not prove to be the case here."

The decision on the Mack case came two weeks after Attorney General William French Smith told a District bar meeting that restraints between a manufacturere and a distributor "are not necessarily anticompetitive."

In that same address, Smith outlined in detail for the first time the administration's antitrust policy. He said the Justice Department was revising 13-year-old merger guidelines and was reviewing 1,200 antitrust judgments and decrees to determine which of those should be vacated or modified.

"We must recognize that bigness in business does not necessarily mean badness -- and that success should not be automatically suspect," Smith told the District bar audience.

The second case dismissed yesterday was filed last July, seeking to block a 1979 acquisition of Glen-Gary Corp. of Reading, Pa., by Ibstock-Johnson Ltd., an English company. The merger combined two of the leading producers of facing brick, used for wall exteriors and partitions, in a 13-state northeastern market.

The two companies accounted for a total of about 20 percent of their market.

"Although market shares to 6 and 14 percent are sufficiently large to make a merger one of concern when it occurs in a well-defined market that can fairly be described as concentrated, further analysis of the available facts revealed the level of concentration in the market involved in the present case was too low to support any prediction that the merger would serve to lessen competition," Baxter said.

While the administration decisions yesterday are futher indications of a new set of antitrust policies, Baxter also has vowed to fight effort within the Reagan team to shut down the largest government antitrust suit in history -- the seven-year case seeking a breakup of American Telephone & Telegraph Co.

Defense Secretary Caspar Weinberger has urged Reagan to drop the case and Commerce Secretary Malcolm Baldrige has said the administration opposes the Justice Department's stated goals in the case AT&T's divesting of its long-distance and equipment subsidiaries. A White House level decision on the AT&T case is expected soon.

The only cases brought by the antitrust division since Baxter took over last spring were cases stemming from an antitrust probe of bid rigging in road building initiated by the Carter administration's antitrust chief, Sanford Litvack.