Abstract The activity of cyclic GMP phosphodiesterase was determined using a three step procedure. In the first step, cyclic GMP phosphodiesterase catalyzes the conversion of cyclic GMP to 5′-GMP. In the second step, a known amount of ATP and guanylate kinase are incubated with the 5′-GMP formed in the first step. The amount of ATP which remains is inversely related to the amount of 5′-GMP formed. In the third step, the concentration of ATP is measured using the firefly luciferin-luciferase technique. The validity of the assay is confirmed by its ability to show the linearity of the cyclic GMP phosphodiesterase reaction with respect both to time of incubation and concentration of tissue. It is capable of detecting less than 5 pmoles of 5′-GMP in 150 μl, and can be used to measure cyclic GMP phosphodiesterase activity in a supernatant fraction of rat cerebrum which contains less than 25 ng of protein. It has been used to determine the activity and properties of cyclic GMP phosphodiesterase in unpurified supernatant and particulate fractions of several tissues of the rat, as well as in highly purified fractions of rat caudate nucleus.