Secretion of nectar in unpollinated flowers of Asclepias syriaca clones, located in northern Michigan and northwestern New York state, began after sepal reflexion, and reached a maximum on the second day of a 3 to 6 day production period. A diurnal secretion pattern occurred with peak rates between 0400 and 0800 hr. At any specific sampling time, sugar concentrations of nectar samples were relatively constant in bagged flowers (although affected by microclimatic variables). Nectar volumes varied more widely. Nectar secretion was probably affected by several factors of the microhabitat, especially air temperature, soil moisture, and solar radiation. Total lifetime sugar production for single flowers ranged up to 7.5 mg with mean peak 24 hr yield of 2.8 mg. More sugar was produced in the Michigan plants than those in New York. The total amount of sugar produced was not affected by frequency of nectar removal. Available nectar (standing crop per flower) in midseason averaged 1.3 ul, containing 0.3 mg sugar, but varied with flower age, time of day, microcimate, and visitation by nectar feeders. Analysis of nectar samples by high performance liquid chromatography (HPLC) revealed a sucrose-dominant nectar with an average sucrose/hexose (S/H) ratio of 8.8. The S/H ratio decreased with flower age at both sites. A difference was noted in plant morphology with New York plants having more umbels and seed pods per stem than those in Michigan. This, combined with temporal weather patterns and edaphic factors, may have been responsible for the site-to-site differences found in nectar production. Most of the nectarivores that fed at A. syriaca took nectar during the day (96% between 0600 and 2000 hr). Wasps (Vespidae and Sphecidae) and bumblebees (Apidae, Bombus) were the most abundant visitors in northern Michigan and made up 25% of the attracted clientele. In New York, honey bees (Apis mellifera L.) were the principal nectar feeders (57% of the total).