Some of the effects of GTP on human fat cell adenylate cyclase activity were studied. This nucleotide caused a dosedependent inhibition of basal activity without affecting the hormone-activated rate of cAMP formation. Maximal effects were observed at GTP-concentrations of about 1×10-4M. The relative extent of hormonal stimulation was about 1.5-fold increased in the presence of 0.1 mM GTP.