The metabolism of L-canavanine and L-canaline were investigated in larvae of the tobacco budworm, Heliothis virescens [Noctuidae]. H. virescens larvae were treated with L-[1,2,3,4-14C]canavanine or L-[U-14C]canaline with sufficient cold carrier to provide 5 mg g-1 canavanine or a molar equivalent of canaline (3.81 mg g-1). The preponderant catabolite in both canavanine- and canaline-treated larvae was [14C]homoserine. Other minor metabolites derived from canavanine included [14C]aspartate/asparagine, [14C]glutamate/glutamine, [14C]2-aminobutyrate, [14C]ornithine, [14C]proline, and [14C]isoleucine. Canaline yielded [14C]glutamate/glutamine, [14C]aspartate/asparagine, and [14C]-2-aminobutyrate. Our current studies support the belief that this destructive insect tolerates L-canavanine and L-canaline because of its ability to reductively cleave these potentially insecticidal natural products to L-homoserine and guanidine or ammonia, respectively.