The effects of feeding 2 protein hydrolysates, one prepared by controlled pepsin and pancreatic protease (including elastase II) hydrolysis of milk proteins (PPPH) and the other a di- and tripeptide bacterial protease hydrolysate of bovine albumin (DTPH), on the growth, nitrogen balance and small intestine adaptation of growing rats were analyzed. Two groups of 3-week-old rats (8 rats/group) were fed the liquid diets ad libitum for 2 weeks. The diets had the same caloric, nitrogen, carbohydrate and lipid contents. The amino acid compositions fulfilled the needs of growing rats. The diet differed in the original proteins, the hydrolysis technique used and the molecular weights of the peptides. Nitrogen intakes were similar. Although there was no difference in weight gain, nitrogen balance was significantly higher in the rats fed the PPPH diet (day 4-day 6:PPPH, 60 +/- 4%, DTPH, 25 +/- 5%; day 12-day 15: PPPH, 58 +/- 3%; DTPH, 30 +/- 5%). The stool nitrogens were identical, suggesting improved nitrogen storage in the rats fed the PPPH diet. Small intestine adaptation showed that the rats on the PPPH diet had significantly more protein (mg) and DNA (microgram) per 10 cm of the jejunum (PPPH, 25.6 +/- 2, 393 +/- 20; DTPH: 15.7 +/- 2, 258 +/- 23) and sucrase-specific activity and per microgram of DNA (PPPH, 133 +/- 5.7, 9.7 +/- 0.5; DTPH, 113 v 5, 7 +/- 1). The N-aminopeptidase-specific activity was the same in both groups.(ABSTRACT TRUNCATED AT 250 WORDS)