The use of radioactive salt solutions in ligation experiments has revealed that the larvae of both Zygoptera and Anisoptera absorb Na+ and CF ions through the rectum from highly hypo-osmotic concentrations. In Zygoptera, there are 3, in Anisoptera up to about 500 epithelial pads in the rectum which show fine structural features of transporting epithelia. Rectal ventilation provides contact of these epithelia to the external medium from where the ions are absorbed. The results of histochemical chloride precipitation in conjunction with X-ray analysis, autoradiography, and liquid scintillation counting suggest that these pads, termed rectal chloride epithelia, are the sites specialized for ion uptake. Activities of Na/K- and HCOj-ATPase are biochemically demonstrated in homogenates of the rectum of Aeshna cyanea. The highest activities of both enzymes are found in the membrane fraction. Long-term acclimatization of larvae to different environmental salinities results in different sizes of the chloride epithelia and concomitant changes of ATPase activity. The significant increase in epithelial size and activities of the transport ATPases in highly hypo-osmotic, nearly salt-free medium is consistent with the enhanced osmoregulatory activity required under these conditions.