Analysis of changes in osmolarity of the serum after various doses of hypertonic radiographic contrast media, following a single injection, showed an initial rise of about 35 m/mosm/1, followed by a rapid fall in osmolarity with a subsequent short hypo-osmolar phase. With repeated injections, similar changes are found if the intervals between the injections are long enough. Following repeated injections over a short interval, there may be a more prolonged phase of hyperosmolarity. In this phase of delayed dehydration, there may be clinical complications. Possible forms of treatment are discussed.