The ability of the guanine nucleotides (Gpp(NH)p and GTP-gamma-S) and of fluoride to stimulate adenylate cyclase is equal in the white adipocyte of lean and ob/ob mice. This suggests that the interaction between the guanine nucleotide regulatory component (N) and the catalytic unit of adenylate cyclase is normal. In the membranes from obese mice, the addition of agonist did not diminish the concentration of guanine nucleotide required for half-maximal activation, as it did in membranes from lean mice, indicating that the interaction between the receptor and N may be altered in the obese mouse. GDP-beta-S and GTP were found to be equally potent in inhibiting Gpp(NH)p-activated adenylate cyclase activity in both groups. Experiments with membranes loaded with or depleted of GDP showed that GDP did inhibit the activation of the cyclase under all conditions tested. Although depleting the membranes of obese mice of GDP improved their response to isoproterenol, it did not restore the response to levels seen in the membranes of lean mice. The data show that it is not likely the binding of GDP which limits cyclase stimulation by agonists. The defect must therefore reside in another property of N.