NYSE OKs After-Hours Trading Sessions : Securities: If approved by the SEC, the new times would be the first steps toward round-the-clock activity on the exchange.

The New York Stock Exchange on Tuesday approved two stages of its plan to implement 24-hour trading by the end of the decade.

Approved were two trading periods, called crossing sessions, that would allow completion of trades after hours.

The NYSE board also approved a series of rules for trading after the exchange’s regular hours of 9:30 a.m. to 4 p.m.

Big Board president Richard A. Grasso said the plan “reflects the NYSE’s commitment to innovative services for its customers and positions the exchange competitively in the evolution of the 24-hour marketplace.”

NYSE Chairman John J. Phelan Jr. unveiled the extended trading plan earlier this year in response to increasing competition from other markets.

The first stage of the exchange’s five-part plan was introduced Aug. 3, permitting some late trades at the market closing price for orders already in place.

The two after-hours trading sessions approved Tuesday would be instituted pending approval of the Securities and Exchange Commission.

The first session would run from 4:15 to 5 p.m. and permit trading at the market’s closing price. All orders placed during this session would be matched up and executed at 5 p.m.

The second session, running from 4:15 p.m. to 5:15 p.m., would allow traders to execute computerized program trades based on aggregate prices tied to a market index. The NYSE said the fourth phase of its plan, targeted for 1991, would allow auctions several times during the night of all listed securities. The fifth phase would provide for conventional trading round the clock.

Phelan has said 24-hour trading is planned for the end of the decade, but other officials have said it could be in place within five years.

Round-the-clock trading now exists in the foreign currency and U.S. Treasury bond markets, where dealers conduct transactions via telephone and computer links. The two Chicago futures markets plan to begin computerized, extended-hours trading in November.

Also Tuesday, the NYSE board approved the formation of a market for trading unregistered securities under a new federal regulation. The NYSE said the new market could be in place by early 1991.

The American Stock Exchange also has announced plans for trading unregistered securities, which are placed privately with big institutions. The SEC passed Rule 144a earlier this year easing trading of such securities.