The increase of sucrase activity in homogenates of jejunum and ileum of suckling rats after cortisone administration has been investigated. Serial tissue sections of villi and crypts were also assayed for sucrase activity and these results were compared with the migration of cells labelled with [(3)H]thymidine along the villus. By using a low dose of cortisone (0.5mg/day per 100g body wt.) it was found that the sensitivity of the small intestine producing system to cortisone stimulation increased during the suckling period. On the other hand, 5mg of cortisone/day per 100g body wt. produced practically the same increase of sucrase during the entire suckling period. Sucrase activity in homogenates of the entire small-intestinal wall was first detected 24h after the first injection of cortisone (5mg/day per 100g body weight) to 9-day-old animals and maximum activity both in the jejunum and ileum was reached by 120h. Jejunal activity was greater than ileal activity, but the rate of the increase was similar. The half-time of the increase was 23-27h, whereas enterocytes migrate from the base to the tip of the villi in approximately 72h. Comparison of sucrase activity in serial tissue sections of villi and crypts at various times after cortisone treatment showed that the leading edge of sucrase activity proceeds toward the tip of the villi at the same rate as the advancing edge of newly formed cells. Sucrase activity increased in the newly induced cells as they migrated to the tip of the villi. It was concluded that the increase of sucrase activity in suckling rats after cortisone stimulation is due to at least three factors: (1) increase of activity in newly differentiating cells, (2) increased percentage of villus cells with sucrase activity and (3) continued production or activation of sucrase activity as the cells migrate along the villi.