A quick review of absorption was listed before the lesson on distribution was given. The definition of distribution as it relates to exogenous chemicals was stated. Factors which affect the distribution, include protein binding, solubility, extracellular fluids, and intracellular fluids were discussed. The brain-spinal cord relationship was shown and cerebral spinal fluid explained. The action of the blood-brain barrier in preventing certain drugs from reaching the central nervous system was discussed. The reason that an antihistamine like Benadryl was useful as a sleeping agent but not an antihistamine like Claritin was explained in terms of passing through the blood brain barrier and entering the central nervous system.