The studies presented here investigated the obtainable flows of different contrast media (Iopromide 370 mg iodine/ml, ZK 119 095 370 mg iodine/ml, ZK 139 129 370 mg iodine/ml, Iopamidol 370 mg iodine/ml, Iopromide 300 mg iodine/ml, ZK 119 095 300 mg iodine/ml, ZK 139 129 300 mg iodine/ml, Iopamidol 300 mg iodine/ml, aqua dest.) in 4.1 Charrière coronary catheters. The measurements of the flow achieved by a standardised power of 100 N show that the highest values are reached with the substance ZK 119 095 (both for 300 mg iodine/ml and 370 mg iodine/ml). On comparison of the catheter types there are no differences in the delivery rate. The x-ray contrast-media, however, are significantly different: the lowest iodine delivery rate is found for iopromide 370 with 384.5 mg iodine/s; the highest rate for the test substance ZK 119 095 with 648.9 mg iodine/s. Although contrast media with low viscosity contain considerably less iodine/ml it is possible to achieve an iodine density in coronary vessels by about 86% higher than that achieved by contrast media with 370 mg iodine/ml. Therefore, the possibility to choose a viscosity-adapted x-ray contrast-medium allows the use of very thin cardiac catheter systems without leading to a worsening of picture quality.