Ionized serum calcium was determined, using a selective electrode, after administration of 80 ml of radiological contrast medium. The ionized calcium concentration was significantly diminished during five minutes after injection (control: 0,98 +/- 0,02 mmol/l; 3 min: 0,94 +/- 0,02 mmol/l, p less than 0,01; 5 min: 0,94 +/- 0,01 mmol/l, p less than 0.05). Determinations were also performed in vitro on whole blood and separated plasma after addition of contrast medium. These determinations revealed a direct interaction between ionized calcium and contrast medium and the importance of the delay after mixing. Clinical consequences under the conditions of the study are probably null but this secondary effect of the contrast medium can explain incidents or accidents observed during the first local circulation of contrast medium especially during coronarography.