BALTIMORE(AP) Desperate for a victory against their neighbors to the north, the Washington Nationals got just what they needed from their most reliable pitcher and most productive slugger.

John Lannan took a four-hitter into the eighth inning, Adam Dunn hit a colossal home run, and the Nationals defeated the Baltimore Orioles 5-3 on Sunday to avoid a three-game sweep.

Willie Harris also homered for Washington, and Josh Willingham had three hits and scored twice. By winning their interleague finale, the Nationals generated a bit of optimism for their upcoming NL East series against the Marlins.

''It was good to get that W and get on the plane and go down to Florida happy,'' Harris said. ''Hopefully get some of that momentum for (Monday).''

Lannan (5-5) gave up three runs and six hits in 7 1-3 innings. The left-hander went 3-0 with a 2.19 ERA in June; he had never before won three games in one month.

In the process, Lannan helped establish himself as the leader of a young starting rotation that owns a collective 15-27 record.

''I think he is, without a lot of hoopla,'' manager Manny Acta said. ''I think the other kids realize that. He leads by example. He works very hard.''

Lannan has allowed three earned runs or fewer in 12 of his last 13 starts.

''Last year I would have a quality start, make it two in a row, then I would kind of fall off,'' he said. ''I'm finally finding some consistency, which is great.''

Mike MacDougal worked a perfect ninth for his third save.

Washington trailed 1-0 in the fourth inning before Dunn connected on an 0-2 pitch from rookie David Hernandez (1-2) following a double by Ryan Zimmerman . The ball soared over the right-field scoreboard, landed on Eutaw Street - an estimated 442 feet from the plate - before hitting the B&O Warehouse on one bounce.

It was the 50th home run to hit Eutaw Street in the 18-year history of Camden Yards, the first by a Nationals player. Dunn, who came to the plate locked in an 0-for-10 skid, has 19 homers this season and 297 for his career.

For Dunn's teammates, the shot was impressive as it was important.

''It was unbelievable. I didn't even tell him good job or way to swing. You don't have to say anything to that,'' Harris said. ''It was just good for us to get that lead right there and for Lannan to go out and pitch with the lead.''

Washington added a run in the sixth when Elijah Dukes singled in Willingham, who scored after rookie catcher Matt Wieters tried to apply a quick tag and dropped the throw from Adam Jones in center.

''It was one of those things were I knew the guy was dead and Jones made a great throw,'' Wieters said. ''It hit the glove and popped out.''

Harris homered off Chris Ray in the seventh, and Willingham made it 5-1 in the eighth when he doubled, stole third and continued home on a throwing error by Wieters.

In 21 games as Baltimore's catcher, Wieters has caught only two of 15 would-be stealers and has made three errors.

Recalled from Triple-A Norfolk before the game, Hernandez allowed three runs and eight hits in 5 2-3 innings. The homer to Dunn was the first against the right-hander in four big-league appearances.

''I was trying to elevate a fastball and I just put it right down the middle,'' Hernandez said. ''I put it on the tee for him. That's what he does with mistakes - he hits them out.''

The Orioles went up 1-0 in the second inning when Nolan Reimold drew a leadoff walk, took third on a double by Luke Scott and scored on a single by Oscar Salazar .

Baltimore didn't get another runner into scoring position until Robert Andino led off the eighth with a double. Ty Wigginton followed with a single to chase Lannan, and Nick Markakis hit a sacrifice fly off Joe Beimel before Aubrey Huff hit an RBI single. Huff, however, was thrown out trying to stretch the hit into a double.

''We made too many mistakes in a lot of different areas,'' Orioles manager Dave Trembley said. ''That was the game.''

NOTES: Washington finished 2-4 against Baltimore and 7-11 against the AL. The Orioles were 11-7 in interleague play. ... Orioles 2B Brian Roberts received a day off after going 0 for 10 in his previous three games. ... The 10-game hitting streak of Washington's Cristian Guzman ended with an 0-for-5 performance.