The Republican majority leader of the state Senate acknowledged yesterday that he is the subject of a federal investigation concerning his business interests outside the Legislature.

Saying he was not aware of the “full nature” of the inquiry, Joseph Bruno said he is cooperating with FBI officials, who told him he was under investigation in the spring.

News of the probe comes at a pivotal point for Albany lawmakers awaiting a new governor, Eliot Spitzer, who is vowing to clamp down on Albany corruption. Just last Thursday, a Democratic senator, Efrain Gonzalez Jr., became the seventh member of New York City’s delegation to Albany to be indicted on corruption-related charges in the last three years.

The inquiry could also have major political implications for the state Senate, the Republicans’ last remaining redoubt against Democratic advances in an increasingly blue state. Mr. Bruno, who represents Rensselaer County, survived the November election with his majority intact, reduced to three seats from four. With the departure of Governor Pataki, Mr. Bruno has embraced his role of leader of the state party. Tension, however, is growing between Mr. Bruno and a Long Island senator, Dean Skelos, who is said to be eager to succeed the majority leader and at one point may have attempted to organize a coup, a Democratic senator, who asked not to be identified, said.

Adding pressure, Democrats in the Senate are actively trying to recruit Senate Republicans to their party in an effort to try to seize the majority before the 2008 election. “There have been discussions among members within the Senate and party about their interest in making sure they are part of the majority, whether it is Democrat or Republican, and which could bring changes sooner than ’08,” a high-ranking Democratic source said.

Republican senators interviewed before Mr. Bruno’s announcement said the majority leader’s support among the conference is secure and that Democratic efforts to flip them would prove futile. “If any of the Democrats wanted to switch, we would consider it also,” a Republican senator of Long Island, Michael Balboni, said.

Mr. Bruno said he was a subject of the probe, as opposed to a target, a distinction that can be blurry. If someone is a target, there is substantial evidence linking the person to a crime. A subject is not suspected of breaking the law, but is a person whose conduct “falls within the scope” of the probe, according to the U.S. attorneys’ criminal resource manual.

Disclosing his involvement in an FBI investigation at a hastily arranged press conference in Albany in the late afternoon, Mr. Bruno, 77, said news about the inquiry had been leaked to press outlets and he wanted to preempt the imminent coverage by telling his side of the story.

“I wanted to be up-front and assure that I have nothing to hide and avoid speculation, unfounded rumors, and distortions,” Mr. Bruno said, the Associated Press reported. “There have never been conflicts in anything I have done.”

Speculation immediately arose that the probe was connected to state money that Mr. Bruno directed to a nanotechnology firm, Evident Technologies, financed by an investor and major Republican contributor, Jared Abbruzzese, who is under investigation by the state lobbying commission for providing Mr. Bruno discounted rides on his business jet. The New York Times on Saturday reported on the close business links between Messrs. Bruno and Abbruzzese, an investor in one of the entities bidding for the state horse racing franchise. Mr. Bruno, Mr. Spitzer, and the Democratic speaker of the Assembly, Sheldon Silver, are expected to choose a franchise winner next year.

Aides to Mr. Bruno deny the links pose any conflict of interest.

Mr. Bruno said the Legislative Ethics Committee, which is controlled by the legislative leaders, has approved his outside business interests, and he ruled out the possibility that the investigation would force him to step down. Republicans last month elected Mr. Bruno to another four-year term as majority leader, a position he has held since 1994.

In his most recently submitted financial disclosure form, Mr. Bruno listed four sources of outside income: He is the sole proprietor of Troy-based Mountain View Farm, which breeds and raises thoroughbred horses; he is a salaried consultant at Capital Business Consultants LLC, whose Troy address is same as the breeding farm; he is a consultant to the Bridgeport, Conn.-based Winthrop Corporation, an adviser to investment funds, and he also earns income from First Grafton Corp., a real estate development venture. Mr. Bruno and his wife own stock in Exxon Corp., American Electric Power Co., General Electric, MSIF Value Port., Vanguard Institutional Index, Vanguard Primecap, and CalAmp.

“As a part-time legislator, I, and my colleagues are permitted to have outside law practices or business interests,” Mr. Bruno said.

A spokesman for the FBI office in Albany, Paul Holstein, refused to comment and directed calls to the U.S. attorney’s office’s northern district, led by Glenn Suddaby, who did not return a telephone message.