Abstract 1. 1. The aim of this work was to investigate the role of phosphoenolpyruvate carboxykinase (ATP:oxaloacetate carboxy-lyase (transphosphorylating) EC 126.96.36.199) in the conversion of fat to sugar by the cotyledons of seedlings of Cucurbita pepo. 2. 2. The enzyme was partially purified from the cotyledons of 5-day-old seedlings. The Michaelis constants for oxaloacetate and ATP were 56 and 119 μM, respectively. The decarboxylation reaction was optimum at pH 7.4. A range of intermediary metabolites did not affect the activity of the enzyme, but 3-mercaptopicolinic acid at μM concentrations was an effective inhibitor. 3. 3. Centrifugation of extracts of 5-day-old cotyledons sedimented appreciable proportions of the ribulosebisphosphate carboxylase, isocitrate lyase and fumarate hydratase present but very little of the phosphoenolpyruvate carboxykinase. 4. 4. Measurements of phosphoenolpyruvate carboxykinase of cotyledons during germination showed that the maximum catalytic activity exceeded, and changed coincidently with, the rate of gluconeogenesis. 5. 5. 3-Mercaptopicolinic acid inhibited gluconeogenesis from [1- 14C]- and [2- 14C]acetate supplied to excised cotyledons. The detailed distribution of 14C indicated inhibition of the conversion of oxaloacetate to phosphoenolpyruvate. 6. 6. It is concluded that a marrow cotyledons phosphoenolpyruvate carboxykinase is in the soluble phase of the cytoplasm and catalyses a component reaction of gluconeogenesis.