These studies were conducted to determine whether dietary homoarginine, derived from guanidination of synthetic lysine, affects the measurement of apparent and true ileal amino acid digestibility in the pig and to determine the optimal ratio of o-methylisourea (OMIU) to free synthetic lysine in the guanidination reaction that produces homoarginine. In two preliminary experiments, specific amounts of lysine were weighed and added to a solution of 0·6 mol/l OMIU to produce OMIU to free synthetic lysine ratios of 2:1, 1·5:1, 1·4:1, 1·3:1, 1·2:1, 1·1:1 and 1·0:1. The mixture was incubated at a pH of 10·5 and a temperature of 20±1°C for 6 days. The highest yield of homoarginine (proportionately 0·995) was produced with an OMIU to free synthetic lysine ratio of 1·5:1. The serum concentrations of homoarginine and lysine were measured in two test pigs at various times after consuming a single meal containing either homoarginine or lysine. Blood samples were taken before and 0·5, 1·5, 2·5, 4·5, 6·0, 7·5, and 11·5 h after feeding. Thirty minutes after feeding the homoarginine-containing diet, the homoarginine concentration in serum was 0·013 mmol/dl, and increased to a maximum of 0·031 mmol/dl 1·5 h after feeding. Thirty minutes after feeding the lysine-containing diet, the serum lysine concentration was 0·011 mmol/dl and also reached its maximum concentration of 0·025 mmol/dl 1·5 h after feeding. The timing of the changes in concentration of homoarginine and lysine related to feeding were similar, which suggests that homoarginine is absorbed and metabolized in a similar manner to lysine. Six Large White×Landrace barrows, weighing 68·5±3·5 kg and fitted with a simple T-cannula at the distal ileum as well as a jugular vein catheter, were used to determine the effects of homoarginine on endogenous amino acid flows and apparent and true ileal amino acid digestibility. The experimental design was a doubly replicated 3×3 Latin-square design with the three treatments consisting of a lysine-free, zein-based diet and jugular infusion of a lysine solution; a lysine-free, zein-based diet supplemented with homoarginine; and a lysine-free, zein-based diet supplemented with l-lysine. There were no significant differences in endogenous ileal lysine flow for pigs receiving the different treatments (396·6, 407·6, and 360·4 mg/kg DMI, respectively, P=0·71). In addition, with the exception of phenylalanine (P=0·04), there were no significant differences (P>0·10) in the apparent and true ileal digestibility of crude protein and amino acids among the three treatments. Therefore, it would appear that the presence of dietary homoarginine, derived from guanidination of synthetic lysine, does not affect endogenous amino acid losses or the digestibility of crude protein as well as most amino acids. These results are therefore supportive of the use of the homoarginine method as a means of measuring true ileal lysine digestibility.