With the opening of its new Primate Center late last month, following by a year the completion of the Treehouse, an innovative exhibits center for children, the nation's first zoo has taken two big steps toward rehabilitating its tarnished reputation for architectural distinction.

Both projects were done by the Philadelphia firm of Venturi, Rauch and Scott Brown, fitting inheritors of a tradition that began in 1876, when the zoo's high-gabled North Gate was entered for the first time. North Gate was the work of the fabulous Frank Furness, the city's most original (and best) architect of the late 19th century. The idea of hiring premier local architects continued into the 1930s, when the distinguished Paul Cret designed an elephant house in the form of a Pennsylvania stone barn.

There is some irony in this rebirth: The otherwise fine old structures, not to speak of the lamentable buildings of the 1950s and 1960s, were, in effect, prisons for animals. It was this architectural ill-suitedness, as well as declining attendance and revenues, that prompted the Zoological Society of Philadelphia six years ago to embark upon an ambitious building program as part of an overall restructuring of the zoo.

So, as different as they are from each other, the Venturi projects are united in their philosophical distance from the powerful buildings of their predecessors. Each in its way reflects vastly changed attitudes about the proper settings for animals in their confined environments, the place of people in zoos and the role of architecture.

Mary-Scott Cebul, vice president in charge of planning for the zoological society, explains that -- following the lead of the Seattle zoo, but in a much more confined setting (the Philly zoo is 42 acres small) -- she set the basic design guidelines:

"We wanted to create a 180-degree turnabout in the use of the land. Originally, we were a park created for people, who periodically would come across an animal in a cage. About one-tenth of the land was for the animals. Our hope is that we'll make that about two-thirds. We're not going to cage the people, but we're going to constrain them with controlled walkways and controlled views."

In addition to these general aims, the Primate Center had to meet a number of unusual specific requirements. Cebul said she wanted the animals to be higher than the people, to have unlimited access to the building and to occupy as much as possible of the limited site. For visitors, she asked for meandering walkways with viewpoints, screened from one another, all around the center. She pointed out that the building had to be heated for the monkeys, lemurs, gibbons, orangutans, gorillas and baboons, but not for humans, those visiting primates who rarely come in the winter. On these terms the Primate Center is a real triumph: Cebul got everything she asked for.

The solution, a genuinely collaborative enterprise (with James Bradberry as project architect for the Venturi firm and Hanna-Olin of Philadelphia doing the landscape design), was to situate the animals on three mounded, amoeba-shaped "islands" (peninsulas, actually, in that by necessity they are attached to the new building at one end), separated by moats and visible from three outdoor platforms as well as two covered breezeways. The payoff is so much in the viewing -- it is an amazing thrill to see gibbons sporting on the limbs of 100-foot-high sycamore trees -- that unless one is looking for it one hardly notices the sophisticated plan.

The building itself is a long, single-level red brick box punctuated by broad openings for the two breezeways and a central squared-off parapet to hide ventilating equipment. Robert Venturi's touch is most evident in the front fac,ade, which is ever so slightly curved (the radius is 2,500 feet, Bradberry explains) and decorated with a checkerboard pattern of concrete blocks set into the bricks. There are also a serpentine sitting wall and a row of honey locusts. This isn't an awful lot to do to a basic box, but it's just enough to make a plain-jane building likable.

By contrast, the Treehouse is anything but plain. In part this is due to George Hewitt, the 19th-century architect (and one-time Furness partner) who designed the structure as an antelope house in the 1870s -- a high-image, high-gabled chapel, with a central nave for visitors and side aisles for the animals. Hewitt's building, clearly not in keeping with the zoo's 180-degree turn, was slated for demolition, but the architects talked long and hard to convince the clients that it was not only usable, but perfect for a new life as a jazzy hands-on playground for city kids.

Full credits for the transformation would make a long list -- quite a few of the Venturi architects became experts in synthetic materials and all sorts of gadgets while working on the project, and Philadelphia's sculptors and painters outdid themselves in the design and construction of the super-size exhibits. But the lion's share of the credit belongs to Steven Izenour, a Venturi associate whose theatrical flair and contagious enthusiasm are in evidence all over the place.

*It's just a really nice, high-energy job. The restoration is at once sympathetic and economical -- "We were able to do the basic job," Izenour says, "for about the cost of a Butler building a no-frills prefab ." And somehow the new uses make a good fit in Hewitt's spacious nave. The most spectacular of the new exhibits is a massive fiberglass ficus tree that reaches to the rafters, but it's all of a piece with the climb-in beehive, the climb-on dinosaur, the hide-in eggshells and so on.

The wisdom of saving the building is self-evident to the children, who give the place a rough workout. To an adult, it is most apparent at night, when Hewitt's vaults, strung with tiny lights, transform the space into a fairyland. Not surprisingly, it has become Philadelphia's most popular new site for corporate parties.

"We wanted to get away from the mold of the 1950s-style petting zoo," Cebul says. "We thought, 'Let's do something that will advance us a step.' We knew we didn't necessarily need contact with animals. Animals can't take a lot of abuse, and that's a restraint on human behavior. So the idea was to make a place where kids could pretend to be another animal."

She got all of that, and more.