The first practical CT scanner was design0 for the head and it took 4 1/2 minutes to collect data for a pair of cross sections'. Today's scanners take less than 5 seconds to collect the data and are designed to produce cross-sectional images of any part of the body. However, the heart has remained a challenge for CT because of the problems associated with its motion2. To avoid motion problems, much of the early cardiac CT work has been in vitro studies or has examined arrested canine hearts3. Also, gating has been used to produce "stop action" or strobe type images of in vivo beating hearts4. Since gating uses data collected over many heart beats, it can only be used to study periodic phenomena in the steady state. Thus, gating cannot be used in a very important diagnostic area, the study of flow or perfusion using contrast medium or x-ray absorbing dye injected into the blood stream. One group has started construction of a very fast scanner which is designed to scan the entire heart every 10 milliseconds5. This design has had to sacrifice appreciable signal-to-noise ratio (contrast sensitivity) for scan speed, a good trade off for imaging anatomy, particularly in systole when the heart is moving most rapidly, but not necessarily the best choice for transient studies such as the perfusion of contrast medium in the heart muscle, where the signal-to-noise ratio is of paramount importance. Our approach has been to explore the potential of present scanner hardware for transit and cardiac studies. We find, in agreement with most previous work, that the x-ray absorption coefficient of blood and heart muscle are so close to being equal that CT scans without the use of at least small quantities of radiopaque contrast medium are of little value. However, we also find that with the contrast between muscle and blood enhanced by the use of a contrast medium, motion problems are not as severe as had been anticipated and that many useful studies, both static and transient, can be performed with a 5 second scanner. Some examples described here are: assessment of the damaged (infarcted) region following a heart attack, visualization and measurement of abnormal wall motion, regional perfusion in the heart muscle (myocardium) and the condition of coronary bypass grafts.