A base support assembly for the cargo bed of a pickup truck mounted on the truck including a pair of parallel side by side slotted side guide rails installed on a respective cargo bed sidewall and a pair of U-shaped end support frames each having a pair of legs pivotally mounted to each of the opposite ends of the side guide rails so as to able to be swung up to an erect position or lowered down position after the base support assembly is installed on the pickup truck cargo bed side walls. An elongated slotted top guide rail may be connected at either end to the top of each end support frame. The end frames can be collapsed into a support for a rigid tonneau cover by lowering the end support frames.