a hand - held pipettor comprising a piston received in one end of a cylinder , a pipettor tip removably secured to an opposite end of said cylinder , means for reciprocating said piston through a selected stroke to aspirate fluid into and to expel the thus aspirated fluid from said pipettor tip , mutually spaced stop members operatively associated with said manually driven means for defining the range of said stroke and motor driven means for varying the spacing between said stop members to thereby vary the volume of fluid being aspirated into and expelled from said pipettor tip .