Phospholipase D (PLD) has been extracted from rat brain membranes and chromatographically enriched 70‐fold. From the rat brain cytosol, Cdc42 with a Mr of about 24,000 and ADP‐ribosylation Factor (Arf) with a Mr of about 18,000 have been purified to near homogeneity. PLD was activated better by purified cytosolic Arf than by the other small G proteins tested. Cdc42 purified from rat brain cytosol showed 70% of PLD activation activity exerted by cytosolic Arf, suggesting that Cdc42 may be one of the major G proteins involved in the activation of membrane‐associated PLD. While Cdc42 or RhoA exhibited synergistic activation of PLD when administered in conjunction with Arf, Cdc42 and RhoA showed an additive effect when used together. It is possible that Arf and Rho family proteins may have different interaction sites on PLD. These findings support a role for GTP‐binding proteins of the Rho family as well as Arf in the activation of membrane‐associated PLD and further suggest that Cdc42 may be a major G protein involved in the PLD activation in rat brain.