Platelet release has been alternatively viewed as a fragmentation of platelet territories demarcated within the cytoplasm of mature megakaryocytes or as a later event involving segmentation of proplatelet pseudopodia extended from the cell. The mechanical constraints on platelet release were evaluated by measuring the resistance of guinea pig megakaryocytes to aspiration into micropipettes of similar diameter to the width of naturally forming proplatelet projections. Application of increasing negative pressure to the surface of the cells resulted in progressively longer extensions being drawn into the pipette until maximal extension lengths were reached. None of the passively aspirated cytoplasmic extensions fragmented off the cells even at the highest aspiration pressure under physiologic study conditions. The longest extensions were aspirated from megakaryocytes of the most advanced maturation stage, and a proportion of the mature cells yielded very long extensions over 50 mu and up to 150 mu in length. Surprisingly, the ease of aspiration did not correlate to cell size during any stage of maturation. The mechanical behavior of guinea pig megakaryocytes indicates a large availability of surface for extension in mature cells ideal for active proplatelet projection. The lack of mechanical fragility suggests that platelet release is a very late maturational event not yet initiated in the “mature” megakaryocytes available for study from marrow harvests.