A vertical stack of weights is disposed beneath a base member, and at least one selector rod is rotatably mounted on the base member and selectively rotated into engagement with a desired number of the vertically stacked weights. A horizontal stack of weights is disposed on opposite sides of a base member, and at least one selector rod is movably mounted on the base member and selectively moved into engagement with the desired number of horizontally stacked weights. Various combinations of these arrangements may be used to provide adjustable resistance to exercise on dumbbells and/or weight stack machines.