The purpose of this study was to determine the flexure strength (sigma f), Young's modulus (E), and fracture toughness (KIC) of five dental composites after aging in water and air. The composites were, by weight, 75% or 79% glass filler and 25% or 21% resin composed of 60% Bis-GMA and 40% TEG-DMA. The filler was either strontium glass (75Sr or 79Sr) or a combination, by weight, of 90% strontium glass and 10% colloidal silica (75Sr10 or 79Sr10). The specimens, 2 x 4 x 70 mm bars, were aged in either air or distilled water at 37 degrees C and were tested in their respective aging media at a loading rate of 1.22 mm/s. The sigma f and E were tested in four-point loading and the KIC in three-point loading. The addition of the fillers to the unfilled resin resulted in a statistically significant increase in the flexure strength, flexure modulus, and fracture toughness. Aging in air had minimal effect on these properties. However, testing and aging in water led to a significant decrease in the mechanical properties in the first 6 months, but had limited effect from 6 to 12 months.