For the monitoring of gastric emptying, a gamma camera or scanner operating from one side of the patient is subject to variations of counting efficiency due to the changing depth of radioactivity. A double-headed scanner was used to investigate the effects of such changes. Tc-99m and In-113m were used as labels for the solid and liquid components of a meal. It was found that the depth of Tc-99m within the stomach decreased by a mean of 13 mm during the first half hour of emptying. Anterior detection alone underestimated emptying rates by an average of 26%. Death changes also introduced errors into "early emptying" measurements made unilaterally. Such artifacts of measurement may compromise mathematical analyses of emptying patterns.