Theodore G. Venetoulis, 44 year old Baltimore County executive, formally announced his candidacy for governor of Maryland yesterday portraying himself as a reformer untainted by a decade of political corruption in the state.

Although Venetoulis is not widely known in the Washington suburbs, he is an experienced campaign organizer whose own organization for this year's Democratic primary is substantial. Acting Gov. Blair Lee has told associates that he fears Venetoulis' challenge more than any other this year.

Venetoulis, and Lee, are competing with State Senate President Steny Hoyer, Maryland Attorney General Francis B. Burch, Baltimore City Council President Walter S. Orlinsky and former Maryland transportation secretary Harry R. Hughes in the first wide-open Democratic primary in a decade.

He announced his candidacy for governor with a street fair in his old East Baltimore neighborhood and a staff meeting two hours later at the restored Adelpi Mill in Prince George's County.

The two events were well considered for the impressions Venetoulis wished to leave in his formal debut as a statewide politician. Venetoulis, tousle-haired Baltimore County executive, was back before the stone row houses in the Greek section of Baltimore where he grew up, the urban ethnic candidate kissing young girls dressed in bright aprons and spangled scarves, shaking hands with their fathers suited in black.

At the Adelphi Mill in Prince George's County, Venetoulis was the organizer, the reformer and the very serious contender that he is in the Democratic gubernatorial race. He hadn't changed his speech or even the staff people around him.

"We cannot expect the last few descendants of an old breed of politician to embrace the hopes and expectations of our new Maryland," Venetoulis said to the Baltimore crowd of some 1,000 persons. Standing beside him was Pete Twardowicz, chief Internal Revenue Service investigator for the prosecutors who convicted former vice president Spiro T. Agnew and suspended Gov. Marvin Mandel. Twardowicz was not there to endorse Venetoulis but to hand him a boys club plaque in the name of Twardowicz's father, a hero to the youth from the part of Baltimore and a second father to Venetoulis.

"I see a state government that understands that Maryland's most serious problems are related to each other, Venetoulis said. "That a subway in a metropolitan area saves energy for Eastern Shore farmers . . . that a new highway in Western Mayland means more business for the post of Baltimore."

Venetoulis says he has a volunteer organization of 5,000 persons. In all but two of the state's 23 countries there is a Venetoulis staff with a scheduler, press liaison, issue expert and district leader. "Some counties are so specialized they have a special events coordinator," said an aide, Kathy Marx.

Venetoulis has had a part-time staff for one year as well as 10 strategists charged with preparing position papers. The trust is headed by two university professors - one from Johns Hopkins, the other from the University of Maryland.

His campaign has raised $200,000 which is half of the goal set for his primary expenses. "We'll be more modest than the other candidates," his aides say. "We won't spend more than half a million."

Some 10,000 lawn signs have been purchased and about 150 billboards prepared for a burst of Venetoulis media attention next week.

Venetoulis clearly hopes to repeat the pattern that got him elected Baltimore County executive in 1974 against an entrenched political machine. Then County Executive Dale Anderson, along with a former county administrator and the county state's attorney had all been convicted on political corruption charges.

Venetoulis ran an anticorruption slate, emphasizing "openess" in government and using large numbers of youthful volunteers.

This year, he is running in the wake of the conviction of Maryland Gov. Marvin Mandel against a field that includes several asssociates of Mandel, especially Lee and Burch.

"Ted is running against Lee but I'm sure you'll never hear any names," explained his campaign aide Ray White. "Ted will talk about leadership, Maryland's political history and the people can pretty much fill in the names."

His opponents dispute Venetoulis' claim to be an "outsider" in Maryland politics, pointing to his 12 years as a political organizer around the state, sometimes in alliance with political bosses like Irvin Kovens, Mandel's friend and convicted co-defendant.

Venetoulis has managed six major campaigns including the successful race of Baltimore's Mayor William Donald Schaefer.

His entry into the 1974 county executive race was a fluke - he had always been the successful organizer, never considering campaigning himself. He was teaching at Towson State College and asked the president there - James Fisher - to run as the reform candidate with Venetoulis as his manager. The president turned him down and the manager became the candidate - campaigning "like a genius," even his detractors admitted.

The long-shot candidate won and immediately tried to institute his promised reforms. The first year Venetoulis has problems. He was seen too often on television and was dubbed "T.V. Teddie" by reporters there, a name that hasn't disappeared. It took another year before he found his niche.

His administration enacted "sunshine" laws, opening up county meetings to the public, embarked on an ambitious growth management plan, revisied the county's investment policies to save $50,000 a year instituted a new flood-plain program, and a garbage recovery system.

Venetoulis also confronted his first major political controversy: his association with Kovens. During his campaign, Venetoulis denied that Kovens was "involved" in his county executive race.

Almost three years later, it was discovered that Kovens and his friend had contributed some $3,000 to the campaign, a sum Venetoulis regarded as insignificant.