Abstract: A portable knockdown animal pen structure includes three like dimensioned fence or barrier sections that are interlockably connectible to each other to form a free standing three sided enclosure arrangeable intermediate a corral or barn where cattle are contained and a holding chute where the cattle are directed for such matters as vaccinations, blood and pregnancy tests, branding and the like. At least two sections are provided with a gate of which one is used relative to the gate in the corral or barn and the other is movable and releasably lockable to the third section to form two pens or enclosures. In one pen having access to the corral or barn, an animal therefrom is temporarily contained and is denied access to the other pen which has access to the chute and in which an operator is protected from such animal while administering to another animal in the chute.