Its a good thing its free... When I went, Before I had Foursquare, August 21, 2011, there where not a lot of animals, and exhibits where spaced out rather far.
Not a big zoo. If you are looking for a big adventure may have better zoo near home town. Only took about 2 hrs to go through and look at/for the limited animals they actually have.
Smithsonian National Zoological Park is a part of the Smithsonian Institution, the world's largest museum and research complex. The words National Zoo represent a large, complex, and diverse organization with a multifaceted mission demonstrating leadership in animal care, science, and education.