When Agriculture Secretary John R. Block ventures out into the country for a speechmaking tour, he is preceded by security agents who do on a smaller scale what the Secret Service does for his boss, the president.

They check out some of the people Block will be meeting, scrutinize the routes he will travel, examine the rooms he will stay in, look into the background of the local drivers who will transport him, coordinate their investigations with local police, and, when the secretary himself arrives, escort him every inch of the way.

A decade ago, these would have been considered extravagant precautions, but now they are a part of doing business for some Cabinet officers. Virtually all department heads have full-time protective services at their disposals, some more elaborate than others.

When Interior Secretary James G. Watt testifies on Capitol Hill, his path is smoothed by at least one and sometimes two security guards.

A small army of State Department agents escorts Secretary Alexander M. Haig Jr. on his foreign trips. The Environmental Protection Agency is acquiring two personal security officers to guard Administrator Anne M. Gorsuch or her deputy on out-of-town excursions.

The reasons for such security squads are obscure because it is in the nature of protective forces not to discuss their operations. Several of them said they are forbidden to reveal details. Some departmental spokesmen spoke vaguely of threats against their secretaries but declined to discuss them. "Good security begins at home, they say," replied a State Department official when asked for details on Haig's apparatus.

At the U.S. Department of Agriculture, Robert Smith, who is in charge of security in the inspector general's office, acknowledged that there have been no serious incidents involving Secretary Block. There is concern, however, over both international terrorism and "the disillusionment of farmers" in some areas Block visits, he said.

Moreover, the amount of out-of-town protection Block receives is usually based on the dangers judged to lie in the area he is visiting. It is heavier, Smith said, in areas "where people are upset over food stamps or over the grain embargo."

That apprehension apparently also extends to reporters in Washington, who say that when Block holds news conferences in the department, a bodyguard stands nearby scanning the questioners in the audience.

The department's five or six specially trained, in-house agents work at various jobs in the department, and usually only one accompanies Block on routine trips around or outside of Washington. They are attached to a protective operations unit in the inspector general's office and have been trained in security by the Secret Service, at a special Treasury school in Georgia and by the State Department for foreign journeys.

The protection of secretaries is not new with the Reagan administration, although some veteran government officials said recently that personal security has become more elaborate in the past year, possibly because of the attempt on President Reagan's life in March, 1981. It is impossible to gauge whether threats against them are more numerous or more serious than in past administrations.

According to two current officials, the change began in 1970 or 1971 when the Nixon White House instructed the departments to develop their own security programs, pointing out that the Secret Service was supposed to be restricted to protection of the president and the vice president.

Gradually, each department acquired a protective unit. The Justice Department now makes use of FBI agents, and Treasury Secretary Donald T. Regan is guarded by the Secret Service, which is technically part of his bureaucratic domain.

One veteran of past administrations recently recalled the elaborate security surrounding William E. Simon, Treasury secretary in the Nixon and Ford administrations. He traveled the streets of Washington in an armored limousine, similar to those used by presidents and ambassadors abroad, which was preceded by an advance car loaded with Secret Service agents, one of them carrying a machine gun.

Usually only one agent now accompanies the incumbent secretary, Regan, but two were along during a recent excursion to New York that produced a minor row. His shuttle grounded by bad weather, Regan switched to a train and was provided--not at his request, an aide insists--with a private car.

The reason given is that trains, unlike airplanes, are not equipped with metal detectors that spot passengers carrying firearms. Irritated riders, seeing Regan's party of six ensconced in a private car, protested loudly and were permitted to share his accommodations after the train left Philadelphia. Treasury officials are vague in explaining the reasons for such precautions.

"We often get general kinds of threats," said Marlin Fitzwater, deputy assistant secretary for public information, but he added: "I don't know of any recent threats against the secretary himself."

International terrorism and the presence abroad of hostile groups explain the precautions taken for Haig, who by most accounts is the most heavily guarded of any official except the president and the vice president. A large number of armed men and women accompany Haig overseas.

The security force surrounding Defense Secretary Caspar W. Weinberger is considerably smaller, or at least less conspicuous, than that around Haig, according to reporters who have traveled with them.