1. A study of the length, total weight and weight per cm of the small intestine of virgin, pregnant and lactating rats has provided evidence for an increase in intestinal surface area in pregnancy and lactation.
2. Because of such alterations in morphology of the gut the absorption,in vivo, of the substrates studied, glucose and glycine, has been expressed in terms of amount transferred per loop and also per g dry weight of intestine.
3. Using these parameters the results show that pregnancy does not alter the ability of the upper jejunum to absorb glucose and glycine. In lactation there is a significant decrease in the transfer of these substances when expressed per g dry weight of intestine, but not in absolute terms.