This study demonstrates that the Inhibition of adenylate cyclase activity by Ca" is enhanced m the presence of mcreasmg [EGTA] (0, 0.3. 1. 2.5 mM) by 2 orders of magnitude.It has been established that this effect is not because of poor Ca" buffering by low [EGTA] or high Ca*' bmdmg by the membrane preparation.It is present irrespective of stimulus. We suggest the enhanced sensitivity of adenylate cyclase to Ca" induced by EGTA is caused by the Ca-EGTA complex being a more Inhibitory spectes than Ca". Thus consideration of the effects of the CapEGTA complex should be made when interpreting the results from expertments mvolving Ca" and EGTA.