New York ? The brewing stock exchange war heated up Friday as the Nasdaq Stock Market announced that it would buy Instinet Group Inc.’s electronic trading network for $934.5 million.

The announcement, which had been widely anticipated, came two days after the New York Stock Exchange, Nasdaq’s biggest rival, announced that it would merge with electronic trading firm Archipelago and become a for-profit, publicly traded company.

The pair of high-profile deals sets up what could be a long and brutal battle for dominance in stock trading. And it reduces what had been a highly splintered marketplace to two dominant competitors.

While the deals are of enormous significance to the stock trading industry, in which different marketplaces compete for listings and order flow and to provide market data, it remains to be seen exactly what the two mergers will ultimately mean for individual investors.

Some analysts and federal regulators have said the deals should benefit consumers by encouraging greater competition between the NYSE and Nasdaq to produce faster, cheaper stock transactions. Federal securities and antitrust regulators closely will examine both deals.

Nasdaq chief executive Robert Greifeld said, “This transaction will position us to offer investors increased choice in the trading of stocks listed on other markets and it will also provide increased liquidity and time priority for stocks listed on Nasdaq.”

Greifeld said the deal was not a response to the NYSE’s deal with Archipelago.

“We have truly bunkered down to get this transaction done over the last three weeks and the (NYSE deal) really had no bearing on the negotiations that were involved in our transaction. Our transaction was happening regardless of what was happening across the street,” he said.