LINE AND COLOR. Those are the essential ingredients of drawings, whether in prehistoric caves or the contemporary galleries now showing the work of Sol LeWitt and Leo Villareal. The latter artists, however, each introduce elements that were unavailable -- indeed, unthinkable -- to their pre-20th-century forerunners: Villareal adds light and motion, and both he and LeWitt open their art to happenstance.

"Conceptual artists are mystics rather than rationalists," wrote LeWitt in 1968, when his renown was new. Yet his brand of mysticism seemed geometric, even mathematical. Rather than embracing chaos, LeWitt's drawings and sculptures imposed order, using regular shapes, right angles and repeated forms. For LeWitt, whose work was akin more to Bauhaus architecture than the messier (and often more personalized) output of fellow conceptualists, the exemplary form was the grid.

Unlike many conceptual artists, LeWitt makes objects that last. Yet not all of them are designed for physical posterity. His wall drawings are meant to be applied directly to the surface, and can be erased or destroyed and subsequently redrawn. Almost always executed by someone other than the artist himself, these pieces are secondary to the blueprints for making them, which are the fundamental artworks.

As LeWitt has put it: "The idea becomes a machine that makes the art."

That machine was at work from May 10 to May 19 in the concourse galleries of the National Gallery of Art's East Building, where a 1971 LeWitt drawing was installed. The full name of the large piece -- 27 by 16 feet -- might seem to be a complete description: "Wall Drawing #65. Lines not short, not straight, crossing and touching, drawn at random, using four colors, uniformly dispersed with maximum density, covering the entire surface of the wall."

The crucial words there are "not straight." Several other LeWitt pieces have been installed within walking distance of "Wall Drawing #65," but none of them includes that instruction. Since 1993, "Wall Drawing #681" has adorned a space outside the entrance to the East Building auditorium, just steps from the new piece. Across the Mall at the Hirshhorn Museum and Sculpture Garden, "Wall Drawing #1113" and "Wall Drawing #356 BB" were executed in 2003, joining a LeWitt sculpture in the top-floor gallery that's the Hirshhorn's only room with a view. (Also, "Wall Drawing #1103" was installed last year at the Washington Convention Center, but access to that one is no easy matter.) All these drawings feature straight lines and hard edges, usually with solid blocks of color. Even "Wall Drawing #681," whose hues are richly (and beautifully) mottled, gives an overall impression of strict regularity.

So "Wall Drawing #65" comes as a surprise. Drawn by four people, including LeWitt assistant Hidemi Nomura, it's remarkably loose and unexpectedly pastel-toned. These tendrils of red, yellow and two shades of blue are not very assertive from a distance. (Indeed, it's easy to walk right by the artwork, which is in a corridor to the left of the room you enter upon descending the half-flight of stairs that starts near the other LeWitt installation.) Are the pencil lines sufficiently packed to qualify as "uniformly dispersed with maximum density''? When a different version of the same piece appeared at the San Francisco Museum of Modern Art in 2000, critic Apollinaire Scherr called it "a tangly expanse of dark hair -- a mess."

In its National Gallery iteration, "Wall Drawing #65" is tangled, but not at all dark or especially messy. Some viewers may be disappointed by this version's gentleness, but within LeWitt's rules there's no right way to execute one of these works. "Wall Drawing #65" is not as striking as its neighbor, "Wall Drawing #681," but it's no less perfect an expression of the artist's system. Like the musical compositions John Cage called "aleatory" -- from the Latin for "dice" -- LeWitt's pieces exist as chance expressions of an enduring conception. Talk about making objects that last -- in theory, these two drawings will outlive the walls on which they've been installed. They are exemplars that can endure forever, which is indeed a mystical notion.

Leo Villareal does his own drawings, but he doesn't use pencils or pens. He drafts with light, generated by LEDs (light emitting diodes). Born almost 40 years after LeWitt, the thirty-something Villareal has some newer methods and influences, derived from mathematical theories and computer graphics and games. The central piece in the New York artist's current show at Conner Contemporary Art is "Horizon," whose glowing tubes suggest neon, but are in fact illuminated by LEDs.

Like "Wall Drawing #65," "Horizon" is heavy on pastels and subject to serendipity. Its computer-controlled patterns are not infinite in variety, but they are randomized, so that the piece doesn't simply repeat the same colors in the same cycle of pulses and sustained notes. Entire tubes or groups of tubes may glow with the same hue, but other times they display a variety of colors, or none at all. The effect ranges from cyber firefly (bursts of yellow) to illuminated Creamsicle (orange and white) to a blare of artificial colors that suggests a Tokyo entertainment district rendered by an electronic impressionist. One of the more interesting, if less aggressive, sequences offers a study in extremely pale tints: shades of pink, green, blue and lavender within lights that, if seen separately, would probably be identified simply as white.

The 24-foot-long "Horizon" is actually two pieces, which are linked in the darkened gallery, but which can exist separately. Also on display are two smaller works, "Bulbox 3.0" and "Particle Playground." The fast, randomly dancing white LEDs of the former are less serene, and less interesting, than those of its similarly scaled companion. "Particle Playground" is more seductive, because its blinking lights come in various colors, and are diffused by a plexiglass box. The combination suggests a stained glass window illuminated by rapidly shifting rays of light, especially since the range of colors is heavy on churchly reds and purples. Villareal's art may be inspired by computerized diversions and game theory, but it can summon transcendent feelings.

SOL LEWITT'S WALL DRAWING #65 -- On permanent display at the National Gallery of Art, Fourth Street and Constitution Avenue NW. 202-842-6333. Open Monday-Saturday 10 to 5, Sunday 11 to 6.

LEO VILLAREAL -- Through June 26 at Conner Contemporary Art, 1730 Connecticut Ave. NW. 202-588-8750. Open Tuesday-Saturday 11 to 6.

A view of Leo Villareal's 24-foot-long "Horizon" at Conner Contemporary Art, at left; and an assistant putting Sol LeWitt's ideas into form in "Wall Drawing #65" at the National Gallery of Art.Villareal's "Particle Playground": suggestive of stained glass illuminated by rapidly shifting rays of light.