“This paper presents evidence that the ribonucleotide derivatives of 6-mercaptopurine, 6-thioguanine, %azaguanine, and 4-hydrosypyrazolo-(3,4-d)pyrimidine effect a pseudofeedback inhibition of glutamine ribosylpyrophosphate 5-phosphate amidot.ransferase, mimicking the inhibit.ory action of natural purine ribonucleotides on this enzyme.”