The nucleotide specificity of the cyclic GMP-binding activity in a homogenate of Dictyostelium discoideum was determined by competition of cyclic GMP derivatives with [8-3H]-cyclic GMP for the binding sites. The results indicate that cyclic GMP is bound to the binding proteins by hydrogen bonds at N2H2, N1-H and/or C6 = O, N7, 2(1)-OH and 3(1)-O and possibly via a charge-charge interaction with the phosphate moiety of cyclic GMP. Cyclic AMP only competes with cyclic GMP for binding at a 20,000-fold higher concentration. The same cyclic GMP derivatives were used to modify the hydrolysis of [8-3H]cyclic GMP by phosphodiesterase. The phosphodiesterase is activated by cyclic GMP. The nucleotide specificity of activation of the enzyme differs from the specificity of the enzyme for hydrolysis. This indicates that activation by cyclic GMP and hydrolysis of cyclic GMP occur at different sites of the enzyme. Cyclic AMP neither activates the cyclic GMP phosphodiesterase nor competes with cyclic GMP for hydrolysis. This indicates that intracellular cyclic AMP does not interfere with the action of intracellular cyclic GMP in D. discoideum.