Iohexol (300 mg I/ml) was compared with metrizamide (300 mg I/ml) in pediatric cardioangiography. Forty consecutive patients below one year of age were included in an open, randomized, comparative two-group investigation. Immediate extrasystoles occurred frequently at injections of contrast medium, but the frequency did not differ significantly between metrizamide and iohexol. Both contrast media caused only minor changes in heart rate. Iohexol caused a significantly larger increase in end diastolic pressure following injections in the left ventricle than did metrizamide. The pressure changes registered were, however, moderate in both contrast media groups and the difference between the two contrast media is probably of no clinical importance. A moderate plasma volume expansion was noted after injection of both contrast media. Metrizamide caused a slight increase of ASAT in the 10 min blood sample which differed significantly from that of iohexol, but no increase of enzyme activity was noted after 24 h and the change was considered to be of no clinical importance. Image quality was similar with both contrast media. Iohexol and metrizamide were well tolerated in pediatric cardioangiography.