In 1531, the Nahua Indians of Huejotzingo, a town southeast of Mexico City, sued the Spanish colonial government for excessive taxation. Since the conquest ten years earlier, they'd been forced to furnish Spanish officials with such items as corn, chili peppers, beans, bricks, lumber, and limestone. Although they had paid similar taxes to previous rulers, they believed the colonial government's demands were too much of a burden. As proof they presented the Huejotzingo Codex, an eight-sheet document that relied on drawings and a numbering system to depict the precise amount of goods that they'd turned over--symbolically denoted by bushel, load, or individual. The codex, along with two other documents revealing early forms of written communication by Native Americans, is on display until February 25 at the Field Museum of Natural History, Roosevelt at Lake Shore Drive, as part of the exhibit "In Their Own Voices." The museum is open daily from 9 AM to 5 PM. Admission is $5, $3 for students, seniors, and children. Call 922-9410 for more information.