We have studied the reliability of serum and urinary immunoreactive anionic trypsin (irAT), immunoreactive cationic trypsin (irCT), and amylase activity as rejection indicators in a porcine whole-organ pancreaticoduodenal transplantation model with exocrine drainage to the urinary bladder. No immunosuppressive therapy was administered. Exocrine tissue integrity and function were studied by measuring these enzymes in serum and urine. Urine analyses were performed before and after an intravenous secretin-cholecystokinin stimulation. Of 16 transplanted pigs, 10 became diabetic during a 2-week observation period while six remained normoglycemic. Serum irAT was found to predict rejection while serum amylase and serum irCT did not. An increase in irAT was seen in rejecting pigs preceding the onset of hyperglycemia by a median of 2 days (range 1-9). Secretion of irAT into the urine remained high during the observation period in nondiabetic pigs while the output declined in diabetic pigs. This decline was seen after the increase in serum irAT. When urine was sampled after a secretin-CCK stimulation, these findings were clearly evident, but less unequivocal results were obtained without stimulation. IrAT measurements were superior to measurements of amylase, irCT, or bicarbonate. Thus rejection of a pancreatic allograft was first indicated by a temporary rise in serum immunoreactive anionic trypsin, probably due to the onset of tissue damage. Thereafter, stimulated urinary enzyme output levels gradually declined and finally, hyperglycemia developed.