Large doses of ammonium acetate (500 μg) were injected into the haemocoel of diapause European corn borer larvae, and the ultrastructure of the ileal epithelium was studied at intervals of from 15 min to 48 hr after treatment. The resulting visible effects included the presence of numerous large granules, many cytoplasmic vacuoles, dilated sacs at the base of microvilli, mitochondria with large vacuoles and thick membranes, proliferation of both microvilli and basal infoldings, numerous dense plaques on the cytoplasmic surfaces of microvillar membranes, proliferation of vesicles of agranular endoplasmic reticulum within microvilli, increased numbers of microtubules, and a very large amount of flocculent material in the cytoplasm near the basal cell membranes and in the spaces between the basal infoldings. Within 48 hr after treatment, the cells appeared to be approximately similar to those of untreated insects. It seemed evident that the ammonium acetate greatly stimulated cellular activity. Ileal cells of borers that had been deprived of water for about 2 weeks usually displayed only weak autofluorescence, which correlated with the finding of few small granules and membranous whorls in the cytoplasm. Rod-shaped mitochondria were present in the cytoplasmic regions of the cells and at the bases of the microvilli. There were but few vesicles, microtubules, plaques of dense material, and little flocculent substance. Small cytoplasmic vacuoles were present, and both microvilli and basal infoldings were shortened and reduced. These findings suggested that partial dehydration was accompanied by a reduction of cell activity. Partially dehydrated insects that were given large amounts of water through the anus showed distinct signs of increased activity in the ileum. Granules and whorls of membranes appeared to form rapidly and to be transported through the cell. Large dilated sacs formed at the bases of the microvilli. The microtubule population increased; apparently normal mitochondria could be seen in both cytoplasm and within the microvilli, even to their apices; microvilli and basal infoldings nearly doubled in length; and vesicles and microvillar plaques were again conspicuous. A marker protein, horseradish peroxidase, was administered to diapause borers through the anus into the rectum and ileum. Shortly after treatment, the protein was seen adhering to the intima and to the tips of the microvilli. It was later visible in vesicles and in distinct layers surrounding the microtubules. These results suggest that horseradish peroxidase was taken up at or near the tips of the microvilli, transported through the cells along the microtubules, and was either eventually metabolized by the cells themselves or was secreted into the haemolymph via the basal infoldings.