The endogenous polyamines spermidine and spermine enhanced guanosine 5'-O-(3-thiotriphosphate) (GTP-y-S)-stimulated phosphoinositide turnover with EC 50 values of 100 ± 30 and 50 ± 15 p,M, respectively, whereas the synthetic polyamines N,N'-bis(3-aminopropyl)-1,3-propanediamine and -ethylenediamine inhibited GTP-y-S-stimulated phosphoinositide turnover, with maximal inhibition at 1 mM. Kinetic analysis of GTP-y-S-stimulated phosphoinositide turnover in the absence and presence of spermidine showed that the Km for GTPy-S was not changed (1,303 -} 270 and 1,069 ± 214 nM respectively), whereas the Vrnax was increased by 206% (1,566 ± 141 and 4,792 ± 84 cpm, respectively), indicating that spermidine and GTP-y-S acted at different sites. Spermidine also enhanced Cat+-stimulated phosphoinositide turnover in the absence of GTP-y-S by decreasing the Ca2' requirement of the phosphoinositide-specific phospholipase C. Arcaine and agmatine, polyamine antagonists at the NMDA receptor complex, did not block the effects of spermidine on GTP-y-S-and Cat+-induced phosphoinositide turnover, suggesting that the spermidine effects are not mediated through these specific polyamine sites . Furthermore, spermidine increased the level of [3H]phosphatidylinositol 4-phosphate (ECao = 120 ± 10 j.M), without affecting significantly the levels of [3H]phosphatidylinositol and [3H]phosphatidylinositol 4,5-bisphosphate . Collectively these data indicate that the enhanced phosphoinositide turnover induced by spermidine in the presence of GTP-y-S or Ca 2+ is mediated through multiple levels of the phosphoinositide turnover cascade . Key Words: Sperm idine-GTP-binding protein-3H-Phosphoinositides-Rat brain membranes-Phosphoinositide-specific phospholipase C-Polyamine antagonists .