Those bad guys willing to bend the rules to make a fast buck in the stock market are at it again. Their latest caper, actually a Wall Street oldie that never seems to go out of style: apparent trading on inside information, notably associated with last year’s explosion in mergers and acquisitions, that has already caught the eyes of securities industry regulators.

Regulatory sources tell me the Securities and Exchange Commission and several exchanges, one of which is said to be the Big Board, are conducting investigations and inquiries into the securities of at least eight companies involved in 2004’s M&A deals that rose appreciably in price prior to the official announcement of these transactions.

“It looks like there was a lot of hanky-panky on the M&A circuit,” said one New York Stock Exchange compliance official, who pointed to “a fair amount of suspicious trading.” He declined to elaborate.

All told, 8,337 mergers and acquisitions, valued at $834.1 billion, were announced in America last year, according to the folks at Thomson Financial.

A number of the investigations and inquiries, I’m told, focus on the health care, financial, and telecommunications sectors. Several are said to involve hedge funds.

One former SEC enforcement staffer, now a securities attorney based in Maryland, tells me he’s heard rumblings of several regulatory inquiries into M &A deals in the technology field and thought M&A insider trading abuses could turn out to be the next wave of Wall Street scandals.

One transaction that has come under SEC scrutiny is one of 2004’s biggest deals – Johnson & Johnson’s $25.4 billion, or $76-a share, acquisition of medical device maker Guidant Corp. Though such a marriage had been rumored for a while, Guidant’s shares – without any definitive confirmation of specific talks with J &J – ran up sharply, from just under $50 in July to $72.05, a gain of about 44%, before the transaction was officially disclosed on December 15.

In conjunction with this deal, the SEC, over the last two weeks, is understood to have contacted at least one New York money manager. I spoke to him. Speaking on the agreement he would not be quoted by name, the manager acknowledged being called by the commission, which, he said, inquired about the basis of his Guidant purchase (about 44,000 shares between the mid $50s and low $60s). He said he told the SEC he had no knowledge of J &J’s impending acquisition of Guidant, though he said he had heard such rumors, and insisted to me his purchases were based strictly on “positive fundamentals.” He wouldn’t say, though, what those fundamentals were.

Interestingly, one investment newsletter, Dow Theory Forecasts, in a December 6 commentary, raised questions about Guidant’s fundamentals, which were anything but all positive. In particular, it expressed concern about Guidant’s ability to compensate for the continued erosion in its coronary-stent business. It noted that new drug-coated stents have been taking market share from Guidant’s bare metal stents, and that the company was not expected to have a coated stent on the market before 2006. In the third quarter, Guidant’s volume rose a puny 0.5%, in part reflecting a 38% decline in stent sales. Despite this concern, though, the newsletter recommended Guidant’s stock, in the process pointing to a J&J buyout as a speculative kicker.

One of Wall Street’s biggest traders said he was sorely tempted to sell Guidant short (a bet its stock price would fall) in the low $60s, but he refrained from doing so, he added, because “the buying at the time was awfully strong and looked well informed.” Added the trader: “I’m glad I wasn’t one of the buyers. Who needs a trip to Washington?”

It’s unclear when J &J and Guidant began their talks or whether they’ve been contacted by any regulatory body. A J&J spokesman said the company would have no official comment on Guidant until it officially disclosed the details of its conversations with the company. But one J &J official tells me Guidant’s stock rise prior to the official announcement of the acquisition “has a real foul smell to it.” Guidant didn’t respond to calls seeking comment.

It’s also unclear whether the SEC’s interest in the J&J-Guidant deal is part of an informal inquiry or a formal investigation. The SEC, as is usually its policy, declined comment.