Modifications in content and lipid composition induced by fasting were examined in fat bodies from adults of Triatominae, Dipetalogaster maximus, Triatoma infestans and Panstrongylus megistus. With fasting, total lipid stores dropped approximately 50% for T. infestans and more than 70% for P. megistus. Total lipids analyzed by thin layer chromatography and fractionated by column chromatography on Unisil showed triacylglycerols as the main component in the three species, although P. megistus showed high levels of diacylglycerols (31-46%). Cholesterol amounted to 8-15%. In diacylglycerol fractions, C16:0, C18:1 and C18:0 fatty acids were detected; their ratio varied with species but it was not dependent on nutritional status. In triacylglycerol fractions C18:1 fatty acid was the major component at different times (48-68%); the ratio of monounsaturated to saturated in this fraction was 1.3, 2.6 and 1.2 for D. maximus, T. infestans and P. megistus respectively. The remarkable drop in lipid stores without noticeable changes in their relative composition would suggest that all types of lipid are used at similar rates. The higher content of diacylglycerols in P. megistus may be associated with the better flight performance of this species.