WASHINGTON(AP) Adam Dunn found the perfect way to foil the lefty-lefty matchup when the Orioles summoned a southpaw to face him.

He hit a drive into the Baltimore bullpen.

Dunn launched two home runs, including a go-ahead grand slam in the seventh inning, for a career-high six RBIs and the Washington Nationals rallied for an 8-5 victory over the Orioles on Sunday.

''(Dunn) showed today the difference that he makes in our lineup and how much we needed him over here,'' Nationals manager Manny Acta said.

With Washington trailing 5-4, pinch-hitter Anderson Hernandez led off the seventh with a single against Chris Ray (0-1). Willie Harris singled, Cristian Guzman sacrificed and Ryan Zimmerman was intentionally walked to load the bases.

Jamie Walker relieved, and Dunn fouled off a 2-2 slider before hitting a shot to left field for his 10th career grand slam. It was his 14th home run this season.

''I was just trying to not pull off and hit into a double play,'' Dunn said. ''In that situation, with a one-run lead and with as many ground balls as I've been hitting, I think they were probably playing the percentages. I can see why they do it, but it's kind of a pride deal and I don't like it.''

To get to Dunn, the Nationals willingly sacrificed a .349 hitter in Guzman to move two runners into scoring position and didn't worry when Zimmerman, who was batting .348, was walked.

''If they're going to walk Zimmerman, good. Let them pitch to Dunn. That's something we haven't had here in the past,'' Acta said.

Orioles manager Dave Trembley defended the decision to have Walker - who had allowed left-handed batters to hit him at a .368 clip - face Dunn.

''That was going to be Walker's guy. ... I thought we had one opportunity in the matchup and that was it,'' Trembley said.

Said Walker: ''(Dunn) did what he's supposed to do. I made one bad pitch that cost us the game, and that's the life of a reliever. It's frustrating. We lost the game. If I get him out, punch him out, there's two outs and anything can happen. So it changes the game.''

Ron Villone (2-0) worked a scoreless seventh for the victory. Joe Beimel pitched the eighth and Joel Hanrahan got the final three outs for his fifth save.

Zimmerman singled leading off the sixth, extending his streak of consecutive games reaching base safely to 42 - matching the franchise single-season mark established by Tim Raines for Montreal in 1986. The overall franchise record is 46, set by Montreal's Rusty Staub in 1969-70.

Adam Jones homered for the Orioles, who finished a 10-game road trip with a 4-6 record.

The Nationals were 2-8 on a 10-game homestand.

The Orioles took a 2-0 lead in the third. Pitcher Brad Bergesen singled for his first major league hit, Brian Roberts walked and Nick Markakis tripled to the wall in left-center.

Aubrey Huff 's RBI single made it 3-0 in the fifth. Austin Kearns and Wil Nieves hit consecutive doubles in the bottom half and pitcher Shairon Martis had an RBI single, pulling the Nationals within 3-2.

Dunn followed Zimmerman's single in the sixth with a two-run shot for a 4-3 lead.

Jones connected in the seventh for his career-high 10th homer, giving the Orioles a 5-4 edge.

Bergesen went 5 2-3 innings, allowing four runs on eight hits. He walked none and struck out two. Martis allowed five runs on seven hits, walked two and struck out two.

Notes: Dunn has two multihomer games this season. ... 1B Nick Johnson was a late scratch from the Washington lineup with a stomach ailment. ... Nationals OF Elijah Dukes , who is on the 15-day disabled list with a left hamstring strain, ran in the outfield before Sunday's game and will report to the team's spring training facility in Viera, Fla., on Monday to continue his rehabilitation. ... Baltimore OF Luke Scott , on the 15-day DL with a left shoulder strain, will begin a rehabilitation assignment with Single-A Delmarva on Monday. He will play left field Monday and serve as the Shorebirds' designated hitter Tuesday.