WASHINGTON -- The Nationals are looking to improve their rotation and have expressed interest in trading for Astros right-hander Roy Oswalt, according to a baseball source.

The Nats have not confirmed that they have interest in the Astros' ace, and Oswalt's agent, Bob Garber, did not return a phone message Sunday.

If the Astros and Nationals can work out a deal, Oswalt, 32, and right-hander Stephen Strasburg would be a perfect 1-2 combination in the rotation. It's unknown what Houston would want in return for Oswalt.

But Washington has some obstacles when it comes to acquiring Oswalt's services. Although he requested to be dealt, he has a no-trade clause and is in the fourth year of a five-year, $73 million contract that will pay him $15 million in 2010 and $16 million next season.

Oswalt's desire to be traded from the struggling Astros is fueled by wanting to play for a contender. At 22-22 entering Sunday, the Nationals are off to their best start since the 2005 season and considered a team on the rise once Strasburg and right-hander Jordan Zimmermann are in the big leagues.

Oswalt is considered one of the best pitchers in Astros history. A three-time All-Star, Oswalt has won 139 games with a 3.21 ERA in his 10 years in the big leagues. He's won 20 games twice and finished in the top five in the National League Cy Young Award voting five times.

Oswalt is 2-6 with a 2.66 ERA and nine quality starts this season.