It has recently been observed that GTP mediates Ca 2+ release from internal Ca 2+ stores. In contrast to effects on permeabilized cells, GTP-dependent Ca 2+ release in isolated microsomes requires the presence of polyethylene glycol (PEG). We have investigated the effects of PEG on microsomal GTPase activity and report that PEG stimulates a high-affinity (K(m) = 0.9 μM) GTPase. The effect of PEG reflect an increase in the V(max) of this activity; no effects on K(m) were observed. The concentration dependence for PEG-dependent stimulation of the high-affinity GTPase exactly mimicked that for GTP-dependent Ca 2+ release. The stimulation of GTP hydrolysis by PEG was specific for the microsome fraction; only small effects were obtained with plasma membrane or cytosol fractions. As observed for GTP-dependent Ca 2+ release, the microsomal PEG-stimulated GTPase was competitively inhibited by the GTP analog GTPγS (K(i) = 60 nM). It is proposed that the PEG-stimulated GTPase may represent an intrinsic activity of the guanine nucleotide binding protein involved in the regulation of reticular Ca 2+ fluxes.