President Reagan plans to enter Bethesda Naval Hospital Friday for removal of a benign growth from his large intestine and for tests to see if other polyps are hidden in his colon, the White House said yesterday.

Removal of the polyp, discovered in a routine examination last March, is scheduled in the hospital's outpatient unit, but the president is expected to remain in the hospital overnight, White House spokesman Larry Speakes said.

Reagan, 74, plans to travel to Camp David Saturday morning, Speakes said. He is to deliver his weekly radio speech from there, another spokesman said.

Speakes said the surgical procedure was being done when "it would best fit into his schedule," not because of worrisome symptoms. "It would be an extremely remote possibility that the polyp will be anything other than benign," Speakes said.

Removal of the polyp is to be done during a colonoscopy, a common procedure in which a flexible instrument containing thin, hollow fibers is used to scan the entire length of the six-foot colon. If others are found, they may be removed as well, Speakes said.

Experts in the field said this is a prudent course of action, and some have questioned why physicians treating Reagan have not performed the search earlier.

Although this polyp and one detected last year were not found to be cancerous, some forms can become cancerous and life-threatening if untreated.

Physicians described the growth discovered in March as a "small inflammatory pseudo-polyp," an inflammation of colon tissue and a type not thought to cause cancer. The March tests also indicated hidden blood in the stool, sometimes an indication of cancer, so Reagan was carefully monitored afterward.

Because the blood test can sometimes be a false alarm triggered by such foods as red meats or by such drugs as aspirin, Reagan was put on a special diet.

Speakes said yesterday that blood tests after the diet switch were negative. Although experts said this was a good sign, they said that polyps may bleed only occasionally and that this does not rule out the possibility that the blood came from hidden polyps.

The procedure Friday will be at the the hospital's gastroenterology division under the supervision of Cmdr. Edward L. Cattau.

It is standard for the patient to cleanse the bowel beforehand by following a special diet for a few days, using laxatives or enemas and restricting intake of food or drink for eight hours before the procedure.

Because of potential discomfort, a painkiller and sedative, but not general anesthesia, will be given before the exam, Speakes said. No temporary transfer of power to Vice President Bush is anticipated, he added.

According to Dr. Nelson P. Trujillo, a Washington gastroenterologist not involved in the case, such patients are generally in "non-decision-making form for two or three hours after the injection," which is commonly a combination of Demerol and Valium, and may remain groggy for 12 to 24 hours.

During the colonoscopy, which normally takes less than an hour, the patient is relaxed and sleepy but able to talk with physicians as they maneuver the black rubber tube through the large bowel, searching the walls for unusual growths.

The small polyp found in March will be removed with a wire snare attached to the viewing instrument. The snare heats up and cuts off the polyp, a procedure known as polypectomy.

Although the colonoscopy is considered safe, there is a slight chance of hemorrhaging or perforation of the colon wall.

Trujillo said serious complications had appeared in far less than 1 percent of the more than 3,000 cases done by him and his colleagues in the decade the technology has been available.

In the March exam, Reagan underwent a flexible proctosigmoidoscopic examination, which located the polyp in two feet of large intestine explored. No signs were reported of a polyp detected in a physical examination in May 1984. That polyp was partially removed and found noncancerous.

Polyps are increasingly common with age. The risk of cancer increases the larger the slow-growing polyps, and some types are more cancer-prone than others.

Although colon and rectal cancers are the second most common cause of cancer deaths in this country, they are highly treatable when detected early.

Because a colonoscopy can occasionally miss polyps, a special X-ray is often ordered, Trujillo said. The X-ray is taken after a barium enema and injection of air, which contrast on the X-ray of the intestine.

In recent physicals, Reagan was also found to have signs of diverticulosis, a condition, common to older persons, in which the colon wall has tiny pouches that may cause medical problems. Otherwise, he has repeatedly been pronounced in excellent health.

To help detect colon and rectal cancer before symptoms appear, the American Cancer Society recommends that a digital rectal exam be performed by a doctor annually after age 40, that a test for hidden blood in the stool be done each year after age 50 and that a proctosigmoidoscopy exam of the lower colon be done every three to five years after age 50.