SummaryThis study was designed to determine the critical difference between rat milk and rat chow with respect to their effects on jejunal sucrase activity during the fourth postnatal wk. Rats were weaned onto special diets on postnatal day 17, and jejunal sucrase was assayed on day 28. A pelleted diet containing lactose as sole carbohydrate did not cause depression of sucrase activity. Sucrase values (μmoles/hr/mg protein) were 10.49 ± 0.81 (n= 15) for the lactose chow and 6.65 ± 0.29 (n= 16) for the sucrose chow. This indicates that the nature of the dietary carbohydrate does not account for the sucrase differences of weaned and nonweaned animals. Likewise, the physical consistency of the diet is unimportant because sucrase values were just as high on a liquid diet (10.91 ± 0.77 μmoles/hr/mg protein;n= 8) as on the solid diets. However, when the relative proportions of carbohydrate and fat in the diet were varied, there were significant effects on sucrase activity; values on a low carbohydrate diet (4.30 ± 0.33 μmoles/hr/mg protein;n= 8) being less than one-half those on a high carbohydrate diet (10.91 ± 0.77 μmoles/hr/mg protein;n= 8).