QUANTICO, Va. — Under the searing sun of one of the worst heat waves in decades, a sweat-drenched Marine second lieutenant stepped from the woods on the base here and reported to an infantry captain standing on a dirt road.


The captain handed the lieutenant a sheet of paper. “Write your name and the time on this card,” the captain said. “You have five minutes to take this portion of the test. Do not use any reference materials. When you are done, return this card to that captain” — he nodded to a huge, tattooed man a few yards away — “and he will tell you what to do next. Begin.”


The lieutenant dropped to the dirt beside other sweaty young officers and removed a pen from his soggy uniform. Another officer, his time up, approached the second captain, who took the card, expressed disgust that the lieutenant had not written his name at its top and pointed him to a laminated sheet of paper displaying a grid coordinate.


That coordinate was where the lieutenant was expected for the test’s next stage. When the lieutenant plotted it on his map, he saw that like many of the preceding stations, it was miles away. He shouldered his pack, slung his rifle and began to jog. The temperature hovered near 100 degrees.


This was one sequence in the Combat Endurance Test, the opening exercise in the Marine Corps’ Infantry Officer Course — one of the most redoubtable male-only domains in the American military. And this session of the course could be the last male-only class. Beginning in September, the corps says, female officer volunteers will participate here, part of a study to gauge the feasibility of allowing female Marines to serve in more extensive combat roles.


Col. Todd S. Desgrosseilliers, the commander of the Basic School, which oversees the course, said he had no special concerns as the course prepares to accept women. “Nothing more so with women than with men,” he said.


“We expect them to be fit enough to go through the course when they get here, just like the men are.”


The 86-day course, which meets four times a year, is called the corps’ most grueling school by its instructors and is intended to screen and train potential infantry officers. Its students are volunteers selected from lieutenants who have completed Officer Candidates School and the six-month Basic Officer Course, which trains all Marine lieutenants to lead provisional infantry platoons and in leadership, tactics, fitness and weapons. That school has been coed for decades.


The experiment at I.O.C. could take a year or more; to obtain a statistically meaningful sample, the corps hopes to observe 92 female lieutenants in coming iterations and then, with information gathered from other studies and surveys, make recommendations about women’s service in so-called combat arms.


The Marine Corps does not expect a flood of women to volunteer for the course, though more than one has for the next round. Women make up only 6 percent of the Marine ranks, and the school’s nature deters many Marines, no matter their gender.


(This reporter graduated from the course in 1988; on the first day, a lieutenant regarded by instructors and peers as one of the most fit students suffered a heart attack and died.)


The current course begins with the Combat Endurance Test, which was added in the 1990s.


Last week’s test began in a classroom after midnight. A captain addressed 96 students, each sitting beside a mock M-16A2 rifle (real rifles are not issued until after the first test) and a backpack loaded with food and equipment.


“Notebooks away,” the captain said. “No notes.”


From this moment on, the captain said, for an amount of time unknown to the students, they would be continuously evaluated. Students who failed would be assigned a noninfantry job.


After a lieutenant completed each leg of the test, the captain said, there would be another instructor who would explain the next task. The test was timed, but the lieutenants would not know how much time was allowed for many events, or over all. This uncertainty was intended to force every student to go as fast as he could, never knowing how much energy and food to conserve.


No one was to help anyone else, the captain said. Speaking was forbidden, except when addressing instructors, with one exception. “If you are injured at any time, it is the only time you will talk to another Marine,” he said. “Legitimately injured,” he added, “As in, ‘I have a bone sticking out of my leg.’ ”