The effect of jejunal sucrase activity of premature weaning (PW) of rats is studied and compared with the effect of starvation. Within 24 hours after PW of rats on postnatal day 16 onto a high sucrose diet, there is a highly significant increase in sucrase activity as compared with that in nonseparated controls. During this time, food intake is minimal, the rats lose weight and there is an arrest of jejunal growth. Rats starved from day 16 onward exhibit the same increase of sucrase activity. Adrenalectomy on day 14, i.e. 2 days before PW, results in 33% mortality on the second postweaning day, i.e. on day 18, and in progressive loss of jejunal protein. Sucrase activity in adrenalectomized prematurely-weaned rats does not differ from the activity of intact suckling controls on day 18. Our experiments suggest that the increase in sucrase activity following PW is not related to food intake, but is primarily mediatd by the adrenal glands. Presented results stress the importance of the endocrine system in the intestinal "adaptive" response to PW in the rat. Endocrine factors should be considered in any evaluation of the effect of nutritional manipulation during the weaning period.