The decline in lactase activity during weaning has been well established. However, its molecuIar mechanism remains to be explored. We studied changes in the expression of lactase in terms of the transcription and translation processes in rat microvillus membrane by Northern blot and Western blot analysis, respectively. To examine the effect of dietary change from a milk to a non-milk diet on the developmental pattern of lactase expression, weaning was prevented by keeping the rats under suckling conditions for 27 days after birth. This treatment only suppressed the extent of decline: while the weanlings showed 17 percent activity compared to that of 4-day-old rats, the prolonged suckling rats showed only 42 percent. The changes in the expression of lactase mRNA and protein were parallel with the change of lactase activity. In other words, the fundamental pattern of significant depression of lactase expression occurred relatively independent of dietary modification.This observation indicates that the regulation of lactase expression is firmly determined at the transcriptional level, and that dietary factor such as the termination of lactose ingestion has only a relatively minor effect The decline in lactase activity during weaning has been well established. However, its studying has been be explored. In this expression of lactase in terms of the transcription and translation processes in rat microvillus membrane by Northern blot and Western blot analysis, respectively. To examine the effect of dietary change from a milk to a non-milk diet on the developmental pattern of lactase expression, weaning was prevented by keeping the rats under suckling conditions for 27 days after birth. This treatment only suppressed the extent of decline : while the weanlings showed 17 percent activity compared to that of 4-day-old rats, the prolonged suckling rats showed only 42 percent. The changes in the expression of lactase mRNA and protein were parallel with the change of lactase activity. In other words , the fundamental pattern of significant depression of lactase expression occurred relatively independent of dietary modification. This observation indi cates that the regulation of lactase expression is determined determined the transcriptional level, and that dietary factor such as the termination of lactose ingestion has only a relatively minor effect