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The stimulatory guanine nucleotide binding protein (Gs) is heterotrimeric ( alpha beta gamma), and mediates activation of adenylyl cyclase by a ligand-receptor complex. The alpha subunit of Gs (Gsalpha) has a guanine nucleotide binding site, and activation occurs when tightly bound GDP is displaced by GTP. Together, GDP and fluoroaluminate (AlF4-) form a transition state analog of GTP that activates Gs. The work of other investigators suggests that AlF4- causes subunit dissociation when it activates Gs. We have observed that in solution AlF4- did not cause Gs subunits to dissociate unless NaCl was also present. The effect of NaCl was concentration dependent (10-200 mM). Omitting F-, Al3+, or Mg2+ prevented the NaCl-induced dissociation of Gs subunits. Na2SO4 could not substitute for NaCl in causing subunit dissociation, but KCl could, suggesting that the anion was responsible for the effect. Gs subunit reassociation occurred when the concentration of Cl- was reduced even though the concentrations of AlF4- and Mg2+ were maintained. The absence of Cl- did not prevent AlF4- binding to Gsalpha. We have concluded that AlF4-, a ligand which is capable of activating G proteins, can bind to Gs in solution without causing subunit dissociation.