We studied physical development, behavioral characteristics, and learning capacity in the offspring of mice immunized with nerve growth factor and bovine serum albumin. High titer of antibodies to these factors in the blood of pregnant females determines high levels of these antibodies in the blood of their pups. These changes modulate physical development, behavior, and learning capacity of rat pups. The effects of these antibodies differed in the strength and directionality. Antibodies to nerve growth factor more markedly retarded physical development, reduced learning capacity, and considerably increased pain thresholds in animals.