The effects of contrast media on left ventricular (LV) relaxation as assessed by the time constant of iso-volumic relaxation have not previously been studied. A new nonionic contrast agent (iohexol) has been shown to have fewer deleterious effects than standard ionic agents. Nineteen patients received iohexol and sodium meglumine diatrizoate (Renografin-76®) in a double-blind, crossover study during left and right coronary arteriography and with simultaneous high-fidelity micromanometer measurements of LV pressure. Neither agent induced significant changes in LV end-diastolic pressure after right or left coronary arteriography. After right coronary arteriography, neither agent produced significant deterioration of peak positive dP dt or ( dP dt) DP40 ( dP dt at a developed pressure of 40 mm Hg). However, after right coronary arteriography both agents caused a transient deterioration in peak negative dP dt and the time constant of isovolumic relaxation (p < 0.05 at 20 seconds after arteriography). After left coronary arteriography, sodium meglumine diatrizoate induced deterioration of systemic blood pressure (p < 0.05), peak positive dP dt (p < 0.01), ( dP dt) DP40 (p < 0.05), peak negative dP dt (p < 0.01) and the relaxation time constant (p < 0.01). These effects were not induced by iohexol. Thus, nonionic contrast media exert negligible alterations on LV function when used for coronary arteriography. The findings are of potential clinical importance in view of the large number of patients with depressed LV function who undergo coronary arteriography.