"Look at April," said artist Warren Cutler. "See how her chin is more drawn out than the others. See how much thinner her face is."

Obligingly, April ambled over to the window, pressed her finely drawn chin up against the glass, and stuck out her tongue.

Cutler used to specialize in portraits of humans. Now his subjects are mostly characters like April, a spider monkey at the National Zoo. Cutler, who has worked for the zoo full time for the past four years, and seven other professional artists will be on hand to help participants at the National Zoo Sketch-In Sunday afternoon from 1 to 3. Anyone who can hold a pencil is encouraged to participate in his free event, which honors the conservation work of artistic advice, the zoo will provide clipboards, paper, sketching pencils and more than 2,000 live unclothed models.The models, however, are not guaranteed to sit still, smile, pose or cooperate in any way.

"Some animals don't like to be stared at," said Cutler, as he walked through the zoo with sketchbook in hand. "It has to do with their way of life in the wild."

A lion-tailed macaque monkey, which had been pacing back and forth in the glass enclosed quarters he shares with his two mates and their offspring, suddenly faced Cutler and bared his teeth.

"That's a threat gesture," said Cutler. "He's telling me that this is his territory."

Animal models, unless they're asleep, won't strike a pose and hold it. But this doesn't faze Cutler. "Painting animals is like painting babies - it takes patience," he said as he waited for a Bengal tiger to turn around and pace in the other direction. To attract the cat's attention, Cutler ducked behind the wall in a game of peek-a-boo. The tiger shot him a disdainful look, but eventually walked as desired.

"A moving animal is sometimes easier to draw," said Cutler. "It shows more feeling; its expressions change. You can see the body structure better, too. See how the tiger's shoulders move back and forth." To draw an animal well, according to Cutler, you have to really look at it: See how the tiger has different markings on one side of its face than on the other.Look at the plush foot pads on the Atlas lion's bony little leg. See the white specks in the Rolaway Guenon monkey's black fur. "The more you draw the more you become attuned to little things," said Cutler.

It also helps to choose an animal you can see well - not a bird high up in a tree. Most important of all, pick an animal that intrigues you. "Breeze through quickly, then think about which animal you'd like to look at again," Cutler advises.

To help you capture an animal on paper, the zoo is mounting an exhibit based on a book called "The Natural Way to Draw," by Kimon Nicholaides. According to the Nicholaides method, you first outline the basic shape of the animal. Then you get the feel of it by doing lots of brisk little "guesture drawings." Put them together and you get a fine-looking giraffe, even if it's supposed to be a zebra.

Artists of all ages are welcome at the sketch-in. "Children are the least inhibited," said Cutler. "Their work is as good as mine because they draw all the details that matter to them."

Selected sketches resulting from this and two future sketch-ins will be exhibited at the zoo in January. For those who prefer to appreciate art rather than participate in it, a slide-illustrated talk on art at the zoo will be held in the education/administration building, near the Connecticut Avenue entrance, during the sketch-in.