The LA Zoo wasn't always in as nice a facility as it is now. The "old" LA zoo was built in 1912 and was in use until 1965, when it moved to its current location just a couple miles away in a different area of Griffith Park. It's open to check out, and is adjacent to a picnic area. There are fenced off areas, but all the fences have giant people-sized holes in them. We thought of it more as: "enter at your own risk."
From the picnic area you can actually approach and walk around what used to be the public viewing area of the bear enclosure.
There is also some sort of old barn, with what I think may have been enclosures for small primates in front of it.
We had about two hours of sun for some fantastic pictures, and then it got dark. And when it got dark, it got delightfully creepy - though, as my friend Enrique wrote, "Alas, sometimes, even the eeriest photos can't capture just how wicked a place feels when you're standing in the middle of it."
For the rest of the photoset, here's a slideshow, or just head over to my Flickr photostream.
You can check out Enrique's as well - his photoset focuses a bit more on the heavy feeling of "urban decay" as you explore the abandoned zoo.
In my photoset, you can pretty much see all the structures that are left to see (there was a fire in the area that occurred well after the zoo was abandoned, that took out a few other structures), but there is definitely more there to photograph, so more visits are in order. But first: this old abandoned hospital.
If you'd like to go check out the old LA zoo, there are some decent directions in the user comments on its Yelp page.