The lipid peroxidation product malondialdehyde is mostly bound to proteins in foods as an N-2-propenal derivative that is released as N-epsilon-(2-propenal)lysine by digestive enzymes. N-2-Propenals have been identified as the major forms of malondialdehyde in urine. To determine whether available lysine can be released from the N-2-propenals of lysine in vivo, two preparations containing N-epsilon-(2-propenal)lysine and N-alpha-(2-propenal)lysine or N,N'-di-(2-propenal)lysine were synthesized using radioactively labeled lysine and were administered to rats by gastric intubation and intraperitoneal injection. Both preparations were absorbed from the digestive tract, although not as efficiently as free lysine, but most of the radioactivity was excreted in urine. The radioactive label was also readily excreted after intraperitoneal injection. It is concluded that the N-2-propenals of lysine are fairly stable in vivo, so that, although they are absorbed from the gut, most of the absorbed material is not metabolized and is readily excreted as nonavailable lysine.