Dallas Merrell, who announced a week ago that he was withdrawing as a Republican candidate for the U.S. Senate, walked into the state elections board here today--the filing deadline for the Sept. 14 primary--plunked down $290 in cash and filed to oppose Prince George's County Executive Lawrence J. Hogan for the Senate nomination.

Merrell, 46, said he decided to get back into the race after former U.S. Sen. J. Glenn Beall Jr. assured him he was not going to make an 11th hour challenge to Hogan.

"With Glenn out, some pieces that hadn't fallen into place did," Merrell said. Merrell said he was in his yard about 9 a.m. today, putting two pigs who had escaped back in their pen, when his wife came outside to tell him that Beall was on the phone.

Moments later, after Beall had reassured Merrell that he was not running, Merrell began a frantic Dialing For Campaign Dollars routine, looking for part of the $150,000 he said he needs for the primary and the right to challenge incumbent Democrat Sen. Paul S. Sarbanes, who already has raised more than $600,000.

"Glenn encouraged me to run and a lot of other people have done the same thing since I pulled out last week. We had closed down our headquarters completely. Now, we'll have to move everything back," Merrell said.

Beall's version of their conversation was slightly different: He said he merely returned Merrell's call of the day before, and that he said nothing that should be interpreted as support for Merrell's campaign.

Nonetheless, Merrell's appearance here, just four hours before the 9 p.m. deadline, was just one of the surprises on a frantic, rumor-filled final day for candidates to file for the 1982 elections in Maryland.

Before noon, some rumors had become fact. State Sen. Mary Conroy, who was appointed last month to fill the seat of her late husband, Edward T. Conroy, for the rest of the year, filed for the House of Delegates. Conroy had pledged to the Democratic Central Committee that she would not oppose former Del. Leo Green, anointed to succeed her husband in the senate, but in the last week she decided to challenge for one of the three House seats being defended by incumbents Gerard F. Devlin, Charles J. (Buzz) Ryan and Joan B. Pitkin.

"This certainly changes the complexion of our race," said Devlin, the safest of the incumbents. "Before it looked like a pretty easy ride for the three of us. Now, it isn't."

Conroy's decision was viewed here as a slap at Ryan. A long-time family friend who managed Ed Conroy's campaign for Congress in 1972, Ryan, like many other county Democrats, last year supported Steny H. Hoyer over Conroy in the special election to fill Rep. Gladys N. Spellman's congressional seat.

"Ed didn't forget that and neither did Mary," said one Prince George's Democrat. "Buzz was their friend for 19 years but all she's thinking about is the 20th year."

Ryan, respected by his colleagues as a solid behind-the-scenes legislator, said, "this just means we have to get out there and campaign harder."

One surprise came moments before the deadline when Prince George's House delegation chairman Robert S. Redding filed to oppose incumbent State Sen. Thomas T. O'Reilly in the Democratic primary. Redding's absence from the delegate race gives his friend Frank B. Pesci a clearer shot at reelection.

Montgomery Sen. Laurence Levitan got some good news: He will not have to run against gadfly Republican Del. Robin Ficker in the fall.

Ficker had talked about challenging Levitan, the chairman of the senate budget and tax committee.

Levitan instead will face state Republican Party chairman Allan C. Levey. The 47-year-old Levey had lobbied for the No. 2 spot on gubernatorial candidate Robert A. Pascal's ticket and had talked at various times about running for Congress and county executive. Although Levey last ran for office 25 years ago in his dental school fraternity (he won), one Republican said "if it's been on the ballot, Allan's considered running for it."

Levitan, whose law firm represents Levey in his dental practice, was delighted by the news. "Now at least I don't have to run a negative campaign," he said. "Allan is a nice guy and I know where he stands on the issues."

In the governor's race, the Republican team of Anne Arundel County Executive Pascal and former congressman Newton I. Steers Jr. got opposition from perennial candidate Ross Z. Pierpont, who has run for governor twice before, and Lloyd W. Reynolds, Pierpont's lieutenant governor candidate. For the Democrats, three slates filed to oppose Hughes and his running mate, Sen. J. Joseph Curran Jr.: Baltimore Sen. Harry J. McGuirk and Lt. Gov. Samuel W. Bogley; Ocean City Mayor Harry J. Kelley and Catonsville attorney Lawrence M. Vincent, and husband-and-wife team John and Alma Amelia Schwartz of Baltimore.

As the doors of the election board here were locked at 9 p.m., Del. Thomas Mooney (D-Prince George's) looked at the hectic scene and remarked: "It's an amazing year. It looks like there are more candidates than there were extras in Ben Hur."