Changes in activities of soluble and insoluble acid invertases and in concentrations of ethanol-soluble carbohydrates were examined during development of the third leaf of oat. The leaves did not contain any alkaline invertases. High activity of soluble invertase, high concentrations of reducing sugar and low concentrations of sucrose occurred in the non-emerged lamina, enclosed by the sheaths of older leaves. In the developing emerged region, activity of soluble invertase rapidly declined to a steady state in the mature leaf. In this tissue, the concentration of total soluble carbohydrate was initially low but increased as the leaf matured and sucrose was always the predominant sugar. The activity of insoluble acid invertase remained relatively constant throughout leaf development. When invertase activities and amounts of soluble carbohydrates were compared in apical and basal regions of the mature leaf after ligule emergence, higher levels of both soluble and insoluble invertase, and total soluble carbohydrates, occurred in the apical half of the leaf.