Abstract A stochastic computer model was used to simulate reduced breeding in populations of sparrowhawk and peregrine falcon. These raptor species have been reported to have been affected by a reduction in reproductive success, caused by organochlorine compounds. For the simulations, literature data on British populations of the two species were used. Reductions in brood size of up to 50% of the control value resulted in smaller but stable sparrowhawk populations. Breeding populations of the peregrine falcon did not decrease when brood size per breeding pair was reduced to 70% of the control value. Increased summer and/or spring mortality and a reduction of the number of successful nests of up to 20% for a six year period was applied to peregrine populations to simulate World War II persecution of this species along the English South Coast. Severe reduction in breeding population size resulted, but recovery was steady, even in populations with a clutch size reduced permanently by 23%. The simulation results indicate that the observed decline of the two raptor species could have been due to causes other than the postulated impairment of breeding success by organochlorines. Disappearance of suitable breeding habitat and increased human pressure upon the countryside in general are likely additional factors.