A reproducible assay for adenyl cyclase activity in an isolated fat cell membrane preparation is described. Enzyme activity was stimulated by epinephrine and sodium fluoride, but not affected by insulin. GTP, GDP, GMP, CTP and UTP inhibited the adenyl cyclase activity of the fat cell membrane, but guanosine and cyclic GMP had no effect. In contrast to the inhibitory effect on the fat cell membrane enzyme, GTP did not affect either adrenal or kidney adenyl cyclase activity. © 1969.