Compositions in the narrative and argumentative modes of discourse were collected from 240 students (40 boys and 40 girls each from grades 6, 10, and 12) and analyzed for mean T-unit length to determine any differences in syntactic complexity across modes of discourse or grade levels. At each grade level, T-unit length was significantly greater in the argumentative mode than in narration. The differences between argument and narration at each grade level were greater than the difference between grades 6 and 12 for either mode of discourse. Significant increases in argumentative mode T-unit length appeared between grades 6 and 10 and between grades 10 and 12; but a significant increase in the narrative mode T-unit length appeared only between grades 6 and 10. These data provide further evidence that syntactic complexity in written composition is affected by task variables and that, while syntactic complexity increases with age, it levels off earlier in narrative writing than in other kinds of writing. (Author/RL)