['Although the molecules involved in mitosis are becoming better characterized, we still lack an understanding of the emergent mechanical properties of the mitotic spindle. For example, we cannot explain how spindle length is determined. To gain insight into how forces are generated and responded to in the spindle, we developed a method to apply controlled mechanical compression to metaphase mitotic spindles in living mammalian cells while monitoring microtubules and kinetochores by fluorescence microscopy.', 'Compression caused reversible spindle widening and lengthening to a new steady state. Widening was a passive mechanical response, and lengthening was an active mechanochemical process requiring microtubule polymerization but not kinesin-5 activity. Spindle morphology during lengthening and drug perturbations suggested that kinetochore fibers are pushed outward by pole-directed forces generated within the spindle. Lengthening of kinetochore fibers occurred by inhibition of microtubule depolymerization at poles, with no change in sliding velocity, interkinetochore stretching, or kinetochore dynamics.'] Does compression regulate mitotic spindle length by a mechanochemical switch at the poles?