36 CFR 520.9: Soliciting, vending, debt collection, and distribution of handbills.

The soliciting of alms and contributions, commercial soliciting and vending of all kinds, the display or distribution of commercial advertising or the collecting of private debts, in or on the premises is prohibited. This rule does not apply to national or local drives for funds for welfare, health, and other purposes sponsored or approved by the National Zoological Park, concessions, or personal notices posted by employees on authorized bulletin boards. Distribution of material such as pamphlets, handbills, and flyers is prohibited without prior approval of authorized individuals.