Mastoparan inhibited [ 3 H]inositol phosphate accumulation induced by carbachol as well as cyclic AMP accumulation induced by isoproterenol in 1321N1 human astrocytoma cells. Mastoparan inhibited GTPγS-induced, but not Ca 2 + -induced, [ 3 H]inositol phosphate accumulation in membrane preparations with an IC 5 0 of approximately 10 μM. The inhibitory effect of mastoparan on carbachol-induced [ 3 pH]inositol phosphate accumulation was resistant to pertussis toxin (IAP) treatment in intact cells. These results suggest that mastoparan inhibits phospholipase C in human astrocytoma cells via a GTP binding protein, which is not a substrate for IAP.