1. Food consumption, live weight, anatomical measurements on the gut organs and the absorptive capacity of the small intestine for L-leucine and D(+)-glucose were made on virgin (control), pregnant and lactating albino rats.
2. Food intake increased by approximately 60% during pregnancy and a further 250% during lactation.
3. Pregnancy did not markedly influence the gross anatomy of the gastrointestinal tract. There was evidence for increased villus height and percentage water in the small intestine and for increased length of the colon during pregnancy.
4. During lactation, the alimentary canal progressively increased in weight and size. It partially regressed following weaning.
5. All anatomical measurements, except the length of the small intestine, completely regressed to control values within 20 d of weaning. The increased intestinal length had not completely regressed by day 30 post-weaning.
6. No significant change was observed in absolute absorption of glucose or leucine during pregnancy.
7. Absolute absorption of leucine and of glucose was increased during lactation. Greatest absorption occurred on the 10th day of lactation.
8. Results for absorption of leucine and glucose per unit length indicated that the ability of the mucosal cells to absorb or the number of absorptive cells/mm had changed during lactation and the post-lactation periods.