High intravenous doses of diatrizoate are known to induce a profound degree of pulmonary edema in the rat. In euhydrated rats, similar doses of iohexol do not induce significantly higher lung weights when compared with nontreated animals. However, for dehydrated rats, intravenous administration of equivalent doses of these agents results in significant pulmonary edema formation with iohexol, and enhanced edema with diatrizoate; the same magnitude of response is not seen in euhydrated rats. These results show that patients susceptible to severe contrast reactions should be well-hydrated and preferably given a nonionic agent when contrast material must be administered.