Abstract: A power unit for medical and like stools and chairs, the first disclosed power unit comprising a post assembly comprising a cylinder, a ram or piston tube which telescopes within the cylinder and a reservoir containing oil under pressure of compressed air, which reservoir is carried by and wrapped slenderly and cylindrically around the lower part of the ram. A dip tube in the reservoir accommodates oil flow across a two piece interiorly and exteriorly sealed plastic control valve disposed in the hollow of the ram, adjacent the top of the reservoir, by which (a) the effective length of the power unit is extended and retracted depending on whether the weight of the user is imposed on a seat carried by the upper end of the ram tube when the control valve is caused to be opened by manual manipulation of a lever pivotally secured to the ram tube above the control valve and (b) any selected length is retained when the control valve is closed.