Neurotensin has many actions on digestive tract motility and secretion and stimulates pancreatic growth. We examined effects of chronic administration of neurotensin on growth of small intestine and colon. Four groups of 10 rats were injected with saline or neurotensin (33, 100, or 300 micrograms/kg) every 8 h for 5 days. The small intestine was divided into four segments of equal length, weighed, and assayed for DNA, protein, and brush-border digestive enzymes. The colon was weighed and assayed for DNA and protein. Neurotensin caused dose-related increases in growth of small intestine; at the highest dose, similar increases in weight (12-20%), DNA (23-35%), and protein content (33-39%) occurred in each segment of small intestine. Maltase, sucrase, and leucine aminopeptidase (but not lactase) contents were also significantly increased after neurotensin, but the largest effects were seen in the proximal small intestine. Neurotensin had no effect on weight, DNA, or protein content of the colon. These results suggest a role for neurotensin in regulating growth of small intestine.