JACKSON, Miss. (AP) — Sen. Trent Lott of Mississippi, the Senate’s No. 2 Republican, plans to resign his seat by the end of the year, congressional and Bush administration officials said today.

Lott, 66, scheduled two news conferences in Pascagoula and Jackson later in the day to reveal his plans. According to the officials, who spoke on condition of anonymity ahead of the announcement, Lott intends to resign effective at the end of the year.

No reason for Lott’s resignation was given, but according to a congressional official, there is nothing amiss with Lott’s health. The senator has “other opportunities” he plans to pursue, the official said, without elaborating. Lott was re-elected to a fourth Senate term in 2006.

Arizona Sen. Jon Kyl, who helped broker a bipartisan immigration bill that went down to defeat this year despite President Bush’s support for it, will run to replace Lott as the Republicans’ vote-counting whip, said spokesman Ryan Patmintra.

Lott’s colleagues elected him as the Senate’s Republican whip last year, a redemption for the Mississippian after his ouster five years ago as the party’s Senate leader over remarks he made at retiring Sen. Strom Thurmond’s 100th birthday party.

His retirement means that Republicans will have to defend 23 seats in next year’s election, while Democrats have only 12 seats at stake.

Mississippi’s Gov. Haley Barbour, a Republican, will appoint Lott’s temporary replacement, who will serve until the 2008 elections, when voters will elect someone to serve out the balance of Lott’s term, which runs through 2012.

Lott’s seat is likely to remain Republican. GOP Rep. Chip Pickering of Mississippi, a former Lott aide who recently announced his retirement from the House, is widely seen as a potential successor. Aides to Pickering said he would withhold comment until after Lott’s news conference.