The effects of low and high osmolality ionic contrast agents on the transplanted heart were studied in 75 consecutive patients. Renal function remained unchanged 24 hours after coronary angiography in both groups. Hemodynamic changes were transient and more pronounced after administration of the high osmolality agent; ECG changes and cineangiography quality were similar after both agents. The manyfold increase in cost of the low osmolality contrast agents may not be justified for use in stable patients after heart transplantation.