Abstract This investigation was conducted to determine the effect of ribonucleoside-5′-diphosphate derivatives of glucose other than UDPG upon the activity of UDPG pyrophosphorylase obtained from etiolated sorghum seedlings. UDPG pyrophosphorylase was isolated and partially purified on DEAE-cellulose. No ADPG, GDPG or TDPG pyrophosphorylase activities could be found in either the supernatant solution after centrifuging it at 30,000g or in the partially purified fraction. High concentrations of pyrophosphate was shown to inhibit the rate of formation of α- d-glucose-1-phosphate from UDPG. This inhibition was not observed when ADPG was added to the reaction medium. Furthermore, ADPG increased the apparent activity of the enzyme at low pyrophosphate concentrations. It was shown that this activation was not the result of activation of an ADPG pyrophosphorylase by UDPG. Other adenine nucleotides (ATP, ADP and 5′-AMP) had no effect on the partially purified pyrophosphorylase. Although GDPG increased the rate of α- d-glucose-1-phosphate formation in the crude extract, it had no effect on the partially purified enzyme preparation.