Kinetic studies of the inhibition of adenine phosphoribosyltransferase by adenine 6′-deoxyallofuranoside and 2′-deoxyadenylate indicate that both compounds bind to free enzyme and to the enzyme–phosphoribosylpyrophosphate complex, although they bind with different relative affinities to each enzyme form. The sites to which these inhibitors bind appear to be different from those to which substrates and products bind. Kinetic and physical studies show that adenosine diphosphate and adenosine triphosphate also bind to several enzyme forms, and that their mechanisms of inhibition of this enzyme are complex.