14C-Labeled aspartate and organic acids were supplied to pea-root pieces to elucidate the pathways of carbon leading to asparagine and homoserine synthesis. The pattern of labeling in the products, and the effects of competing non-radioactive acids on the distribution of 14C from labeled substrates, gave the following results. Supplied aspartate is not converted directly to asparagine or homoserine, but must first enter the Krebs cycle. If aspartate is an immediate precursor of either of these compounds, the synthetic pathways must be compartmented from externally supplied aspartate. Carbon leaves the Krebs cycle as succinate, and is converted to other 4-carbon acids in a metabolic system that is separated from the Krebs cycle and presumably outside the mitochondria. The pathway from succinate to homoserine proceeds via extramitochondrial fumarate and malate, and to asparagine via fumarate. Carboxylation of pyruvate (or a related three-carbon acid) leads to homoserine and asparagine via a symmetrical intermediate permitting equilibration of the two ends of the 4-carbon precursor. About four times more carbon from added pyruvate is converted to homoserine and asparagine via the Krebs cycle than by carboxylation.