Iotrolan 280, the first water-soluble, nonionic, blood-isotonic, dimeric contrast medium, was administered intravenously to 12 healthy male volunteers. In a Phase I study with an intraindividual design in comparison with placebo, four doses between 0.15 and a maximum of 0.9 g I/kg body weight were administered in accordance with the principle of dose titration. The highest volume administered was 270.6 ml. The injection rate was 10 ml/min. The observation period was 5 days, with the exception of thyroid parameters (14 days). Iotrolan displayed good general and local tolerance. The typical side effects known from x-ray contrast media either did not occur or were minor after administration of iotrolan. No allergy-like reactions and no effects on hemodynamic parameters were observed. Although one observation is under discussion no effects of iotrolan on impulse generation and propagation in the myocardium could be confirmed. The blood and laboratory and chemical parameters analyzed showed no differences in comparison with placebo. In the treatment group with the highest dose (= 0.9 g I/kg body weight) there was one subject who showed a transient increase of the urinary glucose concentration and one case of a slight transient increase of the serum chloride concentration. No such side effects were seen in the subjects of the treatment groups with lower doses. All renal functions tests were normal. In this study iotrolan showed excellent tolerance after intravenous injection up to 0.9 g I/kg body weight.