The phenylpropanoid enzyme 4-coumarate:coenzyme A ligase (4CL) is considered necessary to activate the hydroxycinnamic acids for the biosynthesis of the coniferyl and sinapyl alcohols subsequently polymerized into lignin. To clarify the role played by 4CL in the biosynthesis of the guaiacyl (G) and syringyl (S) units characteristic of angiosperm lignin, we generated 4CL antisense Arabidopsis lines having as low as 8% residual 4CL activity. The plants had decreases in thioglycolic acid-extractable lignin correlating with decreases in 4CL activity. Nitrobenzene oxidation of cell walls from bolting stems revealed a significant decrease in G units in 4CL-suppressed plants; however, levels of S lignin units were unchanged in even the most severely 4CL-suppressed plants. These effects led to a large decrease in the G/S ratio in these plants. Our results suggest that an uncharacterized metabolic route to sinapyl alcohol, which is independent of 4CL, may exist in Arabidopsis. They also demonstrate that repression of 4CL activity may provide an avenue to manipulate angiosperm lignin subunit composition in a predictable manner.