When a small amount of diatrizoate meglumine, an ionic iodinated contrast medium, is left in a catheter system before injection of amobarbital sodium or when the contrast medium is intentionally mixed with amobarbital sodium, a potentially dangerous situation occurs. The authors showed in vitro that a dense precipitate forms in this situation. This is due to an acid-base reaction between the relatively acidic contrast medium and basic barbiturate and the subsequent formation of insoluble amobarbital.