Cortisone- and/or thyroxine (T4)-induced changes in jejunal lactase activity and epithelial cell migration were studied to determine the relationship of these two events. In suckling rats given a single dose of cortisone on day 6, jejunal lactase activity increased by 37% and cell turnover rate by 95% 3 days later, whereas T4 alone induced no changes. After cortisone plus T4, jejunal lactase activity decreased by 23% while cell turnover rate increased by 176%. Among all animal groups, the patterns of lactase expression along the crypt-villus (C-V) axis were similar, being low at the C-V junction, increasing to a high plateau at the mid- or third quarter villus level, and decreasing slightly at the villus tip. The calculated epithelial cell age at half maximum lactase expression in cortisone-treated and cortisone plus T4-treated rats was 30 and 53% younger than in control rats. Maximum lactase activity in villus cells was approximately 45% higher in cortisone-treated than in control or cortisone plus T4-treated rats. Parallel measurements of sucrase and lactase activities along C-V axis showed elevated lactase activity at higher villus positions than lead sucrase activity, suggesting that cortisone action occurs in villus cells. Thus jejunal lactase expression may be modulated by 1) adjusting villus cell age required for maximum expression, 2) altering the level of lactase activity in villus cells, and 3) changing the turnover rate of lactase containing epithelial cells.