Galactose-1-phosphate uridyltransferase (EC 2.7.7.10), responsible for the conversion of galactose-1-phosphate (Gal-1-P) to uridine diphosphate galactose (UDPgal) was examined in fruit peduncles of Cucumis sativus L. Two uridyltransferases (pyrophosphorylases), from I and II, were partially purified and resolved on a diethylamino-ethyl-cellulose column. Form I can utilize glucose-1-phosphate (Glc-1-P), while form II can utilize either Gal-1-P or Glc-1-P, with a preference for Gal-1-P. Form I was more heat stable than form II. Both Glc-1-P and Gal-1-P activities of form II were inactivated at the same rate by heating. The finding of a uridyltransferase with preference for Gal-1-P indicates that cucumber may have a Gal-1-P uridyltransferase (pyrophosphorylase) pathway for the catabolism of stachyose in the peduncles. The absence of the enzyme UDP-glucose-hexose-1-phosphate uridyltransferase (EC 2.7.7.12) in this tissue rules out catabolism by the classical Leloir pathway. The incorporation of carbon from UDPglc into Glc-1-P as opposed to sucrose may be regulated by the activities of the uridyltransferases. Pyrophosphate, in the same concentration range, inhibits UDP-gal formation (Ki=0.58±0.10 mM) and stimulates Glc-1-P formation. The ratio of units of pyrophosphatase to units of Gal-1-P uridyltransferase was higher in peduncles from growing fruit than from unpollinated fruit. Modulation of carbon partitioning through a uridyltransferase pathway may be a factor controlling growth of the cucumber fruit.