WASHINGTON — For many high school students, picking a college entrance test has become a multiple-choice question.

The SAT has long dominated the bustling college-prep market. But rival ACT is making inroads, buoyed by a shift in conventional wisdom, which now holds that the tests are of about equal value and that a student would be wise to take both. Colleges are driving the trend because admission officers are spreading the word that it doesn't matter which test students take.

The ascendance of the ACT is a boon to students seeking to impress colleges. The SAT tests how students think. The ACT measures what they have learned. Each is a better fit for some students than others.

"You'll do well on at least one of the tests," said Jordan Kirschenbaum, 15, a junior at Churchill High School in Potomac, Md. He plans to take both.

A decade ago, the SAT reigned supreme on the East Coast. But in the past five years or so, colleges have stated "with unanimity" that they don't care which test students take, said Paul Kanarek, vice president of the test-preparatory company Princeton Review.

Although they operate as nonprofit groups, the New York-based College Board, which owns the SAT, and Iowa-based ACT Inc. have an interest in building market share and maintaining prestige among students and colleges in every state.

"We're growing everywhere, but it's especially dramatic down the East Coast," said Jon Erickson, vice president for educational services at ACT.

"We don't see this as a horse race," said Alana Klein, a College Board spokeswoman. "What's important to us is that students are prepared for and succeed in college."

The College Board recently unveiled an eighth-grade assessment and changed a rule to allow students to report only their best scores from multiple tests. Both moves could be viewed as responses to the ACT, which publishes an eighth-grade test and allows students to choose the scores they send to colleges.

College entrance tests remain vital to the admission process, even in an era when dozens of colleges have waived them as a requirement.

The SAT, introduced in 1926, has evolved from its origins as a quasi-intelligence test for Ivy League applicants. Today, the test spans 3 hours 45 minutes and three sections in reading, writing and math, including an essay. The ACT was introduced in 1959 as an alternative, focusing on curriculum, with sections in English, math, reading and science and an optional essay. It is most prevalent in the middle of the country. The core test, not including the essay, takes 2 hours 55 minutes.

Each SAT section yields a score from 200 to 800; ACT section scores top out at 36. A typical applicant to a competitive college might boast section scores in the upper 20s for the ACT and above 600 for the SAT.

To an extent, the recent popularity of ACT reflects backlash against changes to the SAT. The College Board expanded the exam from two sections to three in 2005. The result was a longer test that some students did not care to take twice.

"They're voting with their feet," said Montgomery County (Md.) School Superintendent Jerry Weast.

Fewer students are retaking the SAT, College Board officials confirmed, adding that the number of repeat customers stabilized last year after declining in 2006.

College Board officials say the ACT's gain is not necessarily the SAT's loss. The number of students taking the SAT nationwide is up 30 percent since 1998. For the ACT, the totals are up 43 percent.

Still, the College Board seems to be fighting back. Last summer, the New York publisher announced that students would soon be permitted to pick their best scores from multiple SAT tests to show colleges. At present, colleges receive all of a student's scores. The change, effective with the Class of 2010, seems tailored to encourage repeat business.

Last month, the College Board also announced a new test to prepare eighth-grade students for the rigors of high school. Promoters said the ReadiStep exam helps create a "college-going culture." Some industry insiders say the test is more about test-taking culture: steering ever-younger students onto the SAT track, a role filled by the Preliminary SAT.

"It's a business move, and it's an intelligent move," Kanarek said.

Klein, of the College Board, said the new test "is completely unrelated to the college admissions process." ReadiStep will be rolled out next fall.

Washington Post staff writer Maria Glod contributed to this report.