We determined along the small intestine of young and adult rabbits the activities of lactase (LPH) and sucrase (Sl), the levels of their cognate mRNA~, and examined the in vitro biosynthesis of LPH and pro.Sl. Lactate activity is low in the proximal 1/3 of the intestine, whereas the mRNA levels are high. However, the rates of biosynthesis of the LPH forma correlated well with the steady-state levels of LPH mRNA in all see~nents, indicating that factor(s) acting post-translationally produce a decline in brush border LPH in the proximal small intestine. These factor(s) are not involved in the processing of pro-LPH to mat ure LPH, since the relative amounts ol'the various l'onns of LPH are almost the same :dong the small intestine. Unexpectedly, we find that also for SI the ratio of activity to mRNA is low in proximal intestine. The biosynthesis ol'pro-Sl ~rrclates with the steady-state levels of its mRNA. Hence, the steady-state levels o1" LPI-I and Sl along the small intestine arc regulated both by mRNA level5 attd by posttranslational factor(s).

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