Scintigraphic gastric emptying measurements were made with subjects supine and upright using a dual-detector rectilinear scanner. Previously reported variations of the depth of activity during the course of a study were again found with both postures. Although there was no significant mean depth change in the group when upright, some individual variations were substantial. Measurements with a gamma camera demonstrated similar changes of depth of stomach contents with seated subjects. The resulting variations of attenuation of the emergent radiation lead to appreciable errors in the emptying rates determined by unilateral detection. In about half the cases the mean movement of a 99Tcm-labelled solid phase marker exceeded 1 cm; such a movement led to an average 20% error in emptying rate determination by an anterior detector. Depth changes of a liquid marker were less marked, exceeding 0.5 cm in half the subjects; this movement gave rise to an average 6% error when 113Inm was used as the tracer.