Objective: The purpose was to investigate the effects of cyclic loading and environmental aging on two dental composites, Restolux^TM (fiber filler) and Micronew^TM (particle filler). Methods: Disk specimens 25 mm in diameter and 2 mm in thickness were machined with a 3 mm notch in the center. Testing: Specimens were aged for 4 months in air, water, saliva, and in a 50/50 by volume mixture of ethanol and water at room temperature in sealed polyethylene containers.  Unaged and aged specimens (5 specimens for each variable) were subjected to cyclic loading at a frequency of 5 Hz. The specimens were cycled for 1, 1000 and 100,000 cycles at a load level approximately 60% of the fracture load for non-cycled specimens. Following cycling, the specimens were tested in compression at a loading rate of 1.27 mm/minute.  Results:  A two way ANOVA followed by a Tukey multiple means comparison test indicated that the 50/50 mixture caused a significant 25% decrease of the fracture toughness of Restolux.  For Micronew, all the specimens that were aged 4 months in the 50/50 mixture failed the 1000 and 100,000 cyclic loading.  Cyclic loading for 100,000 cycles also resulted in an increased failure rate for Restolux and Micronew specimens.  Conclusion:  The significant lowering of the fracture toughness of the dental composites following aging in the 50/50 mixture can be attributed to the breakdown of the resin and/or the silane bond between the filler particles and the resin.  Fracture toughness is susceptible to cyclic loading as seen in the specimens subjected to 100,000 cycles.  Funding was provided by NIDCR grant HHS DE07979.