The visibility and thickness of the pericardium, as depicted by MRI, and the changes of these parameters over the cardiac cycle were determined in 18 normal subjects. Gated, cycled, multisection images were obtained in the transaxial orientation. Using a score-point system for quantification, there was better visualization of the low-intensity pericardial line during systole as compared with diastole (p less than 0.005). Pericardial thickness was 1.2 +/- 0.5 mm in diastole and 1.7 +/- 0.5 mm in systole (p less than 0.001) as measured in a midventricular section in front of the right ventricle; both values exceeded the thickness of 0.4 to 1.0 mm reported for anatomic measurements of pericardial thickness. The layer of normal pericardial fluid present in the pericardial space should also have low intensity, and it likely contributes to the overall pericardial thickness as visualized by MRI. Since MRI is sensitive to the small amount of normal pericardial fluid and depicts its anatomic distribution, it should be valuable in detection and quantification of even small pericardial effusions.