Decoyinine is an analog of adenosine which causes a decrease in intracellular GTP levels by inhibition of GMP synthetase. Decoyinine exerts a stimulatory effect on oaamylase synthesis only in the strain in which the catabolite repression component of a-amylase regulation has been rendered inoperable by the gra-10 mutation. The apparent stimnulation of a-amylase activity in WLN-11 by Decoyinine is not an artifactual effect exerted at the level of enzyme activity itself. It is observed that purine nucleotides, most notably GMP, exhibits a dramatic inhibitory effect on the activity of Bacillus circulans F-2 amylase and a less-pronounced inhibitory effect on B. subtilis aamylase. At the concentration used in the experiments (1.07 mM), Decoyinine has no effect in vitro on B. subtilis WLN-11 oaamylase activity. At the same concentration, GMP exerts a slight inhibitory effect on aamylase activity.