The relationship between polyphosphoinositide and phosphatidic acid (PA) metabolism and Mg-ATP dependent shape and viscosity changes in erythrocyte ghosts from four mammalian species was examined. Ghosts prepared from rabbit, dog, human and guinea pig erythrocytes were transformed from echinocytes to discocytes within 15 min in the presence of 1 mM Mg-ATP at 25 C. In all species these Mg-ATP shape transformations were associated with a 30-45% decrease in the specific viscosity of the ghost suspensions. Mg-ATP induced a second transformation of discocytic ghosts to cup shape forms without a further decrease in viscosity. A considerable species variation in the rates of Mg-ATP dependent viscosity and shape changes and incorporation of 32P into phosphatidylinositol-4' phosphate (PIP), phosphatidylinositol-4'5'bisphosphate (PIP2) and especially PA from Mg-[gamma 32P]-ATP in ghosts was found. However, the rates of Mg-ATP dependent synthesis of PIP and PIP2 and shape and viscosity changes in each species were of the same magnitude. Ca2+ or neomycin strongly inhibited PIP labeling and Mg-ATP shape and viscosity changes in ghosts of the different species. Ca2+ or neomycin usually increased or had little effect on 32P incorporation into PA and PIP2. The possibility that Mg-ATP-induced changes in erythrocyte membrane shape and deformability are dependent on increases in membrane PIP and PIP2 is discussed.