Incubation of Gpp(NH)p-activated adenylate cyclase in the presence of isoproterenol caused the release of bound [3H]-Gpp(NH)p, and the decline of activity to the basal state. The isoproterenol-induced release of the nucleotide was proportional to the decrease in adenylate cyclase activity. Since there is a large excess of Gpp(NH)p binding sites in the membrane, the isoproterenol induced release of Gpp(NH)p, rather than binding of the nucleotide, was used to measure the amount of guanyl nucleotide binding sites coupled to the activated adenylate cyclase. This amount, 1.5-2.0 pmoles/mg membrane protein, is only approximately 1% of the total Gpp(NH)p binding sites, and is about equal to the number of beta-adrenergic receptors in the membrane. Chromatographic analysis revealed that Gpp(NH)p was released from the membrane as an intact molecule. The findings suggest that persistent activation of the adenylate cyclase is due to persistent binding of Gpp(NH)p to the regulatory site, and that this GTP analog is a better activator of the adenylate cyclase than GTP because of its resistance to hydrolysis.