Rat pups that are separated early from their mothers, at postnatal day 15, become hypothermic when subjected to physical restraint on postnatal day 30. Restraint of separated pups also elicits an unusually high incidence of gastric erosions, as well as insomnia and an increase in quiet wakefulness. If hypothermia during restraint is prevented, neither the erosions nor the behavioral responses occur. Rat pups separated at the customary age (postnatal day 22) do not become hypothermic during restraint, and the restraint of such pups is not associated with either gastric erosion or insomnia.