The palmitoylation or S-acylation of at least some G protein alpha subunits is a dynamic process that is regulated in vivo by the activation of associated receptors. Highly purified, myristoylated Gialpha1 and other G protein alpha subunits react spontaneously with palmitoyl-CoA in vitro to form thioesterified proteins. This reaction requires native Gialpha1 and occurs exclusively at Cys3, the same residue that is palmitoylated in vivo. The reaction proceeds to completion, and its rate is roughly equal to the rate of loss of palmitate observed in pulse-chase experiments in vivo. The rate of autoacylation is significantly enhanced by the G protein betagamma subunit complex. Autoacylation may play a role in the dynamic thioesterification of some cellular proteins.