Whole-blood free amino acids were measured in a control group made up of eight healthy women fasted for 12 h and also in eight patients with acute pancreatitis, five patients with acute cholecystitis and seven patients with acute appendicitis. Blood was withdrawn immediately on admission to hospital and again 3 d later following a controlled peripheral parenteral nutrition diet; this is with the exception of the appendicitis group. L-CYSTATHIONINE AND l-methionine concentrations were significantly higher in pancreatitis and appendicitis patients when compared with controls. In the pancreatitis and cholecystitis patients, l-serine concentration was also significantly higher when compared with controls. The l-homocysteine concentration was significantly higher only in the appendicitis group when compared with the control group. l-Cystine concentration was unchanged in all the patients studied when compared with control subjects. The l-methionine : l-cystine ratio was significantly higher and the l-glutamine : l-cystine ratio was significantly lower in all the patients when compared with controls. The blood S-amino acid pattern reflects an impairment in liver transsulfuration pathway during acute abdominal processes. This work supports the idea that the l-methionine : l-cystine and l-glutamine : l-cystine ratios can be taken as good markers to evaluate the S-amino acid metabolism and suggests the importance of using N-acetylcysteine as a required nutrient in these situations.