The effect of tick infestation on reproductive success of mammals was studied by using rabbits bred after they had served as hosts for Dermacentor variabilis . Mating success, pregnancy incidence, litter size and neonatal viability were reduced in tick-exposed rabbits, and the proportional number of abnormalities found in preimplantation-stage embryos was much higher than in unexposed controls. Fecundity was restored by prolonging the interval between tick exposure and breeding or by treatment with an acaricide.