Measurements of photosynthetic assimilation and nectar production provide data for the calculation of energy allocated to the production of floral nectar in the common milkweek, Asclepias syriaca. Four to 37% of daily photosynthate assimilated during blossoming is secreted as nectar sugar. Amortization of nectar cost over the seasonal lifetime of the plant showed that as much as 2.8% of photosynthetically assimilated carbohydrate is routed through the nectaries. In comparison, in a field crop of alfalfa, Medicago sativa, nectar is an energy sink containing as much as 20% of the amount of energy contained in the harvested alfalfa forage, or almost twice the energy yield in the total seed crop. The relative costs of nectar production and other costs such as flowering, seed production, and growth and maintenance are discussed.