We studied the effects of acute and chronic administration of secretin and caerulein, alone and in combination, on RNA and protein synthesis in the duodenum and oxyntic gland area as well as content of DNA, RNA and protein in rats. Secretin, 100 micrograms . kg-1, three times a day for 5 days, was associated with duodenal hypertrophy after the first 24 h of treatment and hyperplasia at the end of days 2 and 4; hypertrophy of the oxyntic gland area was observed only at 4 h after the first injection. Caerulein, 1 microgram . kg-1, also promoted duodenal hyperplasia after 2 and 4 days of treatment. The oxyntic gland area showed hypertrophy only at 4 h after the second injection of caerulein. These data indicate that both hormones can induce duodenal hyperplasia, probably by an amplification of the normal renewal cycle of the epithelial cells. They also demonstrated that growth of the oxyntic gland area is not promoted by these two peptides at the doses studied.