Radiographic contrast media are commonly used diagnostic aids to improve imaging, e.g. in computerized tomography. However, the routine application of these agents may cause adverse allergic/pseudoallergic reactions. In order to understand more completely the underlying mechanisms involved in these reactions, experiments on histamine release bothin vivo andin vitro are necessary. Using canine mast cell suspensions from lung and liver, we have investigated the histamine release caused by six commonly used preparations. The dog is an ideal model for bothin vitro andin vivo studies not only by virtue of its size but also because of its similarity to man with respect to e.g. cardiovascular reactions after drug-induced histamine release. The two non-ionic preparations (Solutrast, Ultravist) released little histamine from both cell types (ca. 4–6%). The ionic contrast media (Angiographin, Hexabrix, Telebrix, Rayvist) dose-dependently released histamine from the liver cells and pulmonary cells (maximum release between 18–35%). The liver cells (the liver is the shock organ in the dog) reacted more strongly to these agents than the pulmonary cells, thus providing further evidence for mast cell heterogeneity and the importance of selecting the appropriate mast cell model for the investigation.