Release of viable retinal pigment epithelial (RPE) cells into vitreous cavity and subsequent attachment to the retina may be the first steps in the occurrence of macular pucker and proliferative vitreoretinopathy (PVR) complicating rhegmatogenous retinal detachment. Gravity and post-operative position of the patient together may influence where the cells settle, and, thereby, the location of subsequent membrane formation. To study the effect of gravity on the location of RPE cell attachment, 3H-thymidine-labelled RPE cells were injected into 12 enucleated pig eyes after vitrectomy performed to create a posterior vitreous detachment. The eyes were then positioned to make either the macula or the inferior retina gravitationally dependent. Radioactivity was later measured from several locations on the retina to indicate the proportion of cells attached in each location. Radioactivity measured from the dependent part of the globe (mean 5985 +/- 1728) was always greater than that from other parts (mean 389 +/- 79). The experiment was repeated in live pig eyes with identical results. These findings suggest that patient positioning may affect the location of cellular membrane formation and subsequent retinal traction.