Rye grass 3'-nucleotidase has been purified to apparent homogeneity on Sephadex A-25 and CM-cellulose columns and shown to hydrolyze 2'-O-methyladenosine 3'-monophohate and 2'-deoxyadenosine 3'-monophosphate 35.8 and 542 times more slowly than the normal substrate (3'-AMP), verifying the importance of the 2'-beta-OH group of the substrate in the overall hydrolysis process. Although neither was hydrolyzed as rapidly as 3'-AMP, both the 2'-O-methyl and 2'-deoxy analogs acted as competitive inhibitors of the hydrolysis of 3'-AMP (K-m equals 0.12 mM), with apparent K-i's of 0.39 and 0.51 nM, respectively. In order to determine the possible susceptibility of naturally occurring ribonucleoside 3'-diphosphates, such as guanosine tetraphosphate (ppGpp), to 3'-phosphohydrolase activities, the 3'-nucleotidase was also employed in the attempted pyrophosphorolysis of adenosine 3'-diphosphate and guanosine tetraphosphate. Neither adenosine 3'-diphosphate nor guanosine tetraphosphate was degraded at a significant rate by the nucleotidase, relative to the normal substrate.