A plan to dramatize the western end of Pennsylvania Avenue - with two new marble pylons, each 10 stories tall - yesterday received the tentative endorsement of the National Commission of Fine Arts.

The design, by architect Robert Venturi, solves a formal problem - how to terminate the avenue - that has stymied teams of planners for the past 15 years.

Symbolically, the avenue links the White House and the Capitol. Visually, however, it doesn't do the job. Though it begins, in grandeur, with the white dome of the Capitol, it seems to fizzle out at the back porch of the Treasury a block before it reaches the White House to the west.

Unlike previous designs, which sought to block the avenue with a huge paved square, or trees, or a $1 million fountain, Venturi's slabs would function as a simple open frame.

Like the rows of trees that flank the "infinite" greenswards of Versailles, or the columns of San Marco beside the Grand Canal in Venice, Venturi's pylons would bring a sense of order to a view that is chaotic. "These frames," he says, "make something positive out of just-space."

As one drove up the avenue, the portico of the Treasury, the shrubbery before it, and the background of tall trees on the White House grounds, would organize themselves into a picturesque green landscape framed by marble on both sides.

But because those slender slabs do not stand beside each other as one reached the first one, that tall frame would "dissolve."

Venturi's scheme for the Pennsylvania Avenue Development Corporation is still in the design stage; it has not yet been approved. But Commission of Fine Arts Chairman J. Carter Brown said yesterday, "It has my personal strong endorsement. I think it is a brilliant solution."

Initial plans for ending the avenue with a new "National Square" called for a huge expanse of paving, the razing of the Willard and Washington hotels, and underground taffic tunnels.

Venturi would, instead, build a playful, modest plaza before the District Building. His pylons would rise from open space that is already there.

Venturi, and landscape architect George Patton, would activate the surface of their plaza with small-scale incidentals - lawns, pools, statues, urns, and stairs and garden walls that, during inaugural parades, might function as seats. In addition, they would pave the plaza with a granite map of Maj. Pierre L'Enfant's original design for Washington.

Venturi yesterday compared his 100-foot-tall pylons, and the plaza's smaller details, to the "giant" and "minor" orders of baroque facade.

"The plaza should look big - and good - from a distance. But it shouldn't be brutal when you get there," he said.

Though sculptor Richard Serra recently conceived a 200-foot-high sculpture for the west end of the avenue, his design has been withdrawn.