Caffeine, given during interfraction interval, abolishes the sparing effect of dose fractionation observed for delay of pupariation in Musca domestica larvae. When given as postirradiation treatment after single exposure, caffeine increases the delay in the synergistic manner. Pretreatment of the larvae with ascorbic acid for 3 h protects from radiation-induced delay whereas pretreatment with caffeine does not have any effect. Combination of ascorbic acid and caffeine pretreatment protects the larvae only at low concentration (0.05%) and not at high concentration (0.1%).