This study was undertaken to determine whether the partial or total replacement of fish meal protein by protein from extracted, commercially available soybean meal would provoke a response in pancreatic proteolytic enzyme secretion in rainbow trout fry. At the end of the growth trial (177 days) the activity of chymotrypsin was determined by an in vivo method based on the oral introduction of N-benzoyl-l-tyrosyl-p-aminobenzoic acid (bt-PABA). Comparative trials were run with fish force-fed with PABA. The excretion index (bt-PABA/PABA) reflected the decreasing rate of pancreatic chymotrypsin activity with increasing amounts of soybean protease inhibitors introduced into the diet. This finding is in contrast to the several-fold increase of trypsin activity of faeces collected from fish fed a soybean-meal-supplemented diet. The growth rate of fish was reduced significantly when 50% of the fish meal was replaced by soybean meal, and 100% replacement resulted in growth arrestment and mortality. Amino acid absorption, particularly of methionine, leucine and threonine, decreased even when 25% of the fish meal protein had been replaced by soybean meal protein, and decreased further at the 50 and 100% substitution levels. It is suggested that the increase in secretion of pancreatic proteases caused by the soybean protease inhibitors does not compensate for the decrease in the efficiency of dietary protein digestion. The apparent contradiction between the in vivo assay of chymotrypsin activity and enzyme activity present in the faeces can be explained by inactivation of the protease inhibitors during digesta passage in the gut and by regeneration of inhibitor-bound trypsin in the posterior part of the intestine.