Proplatelets become shorter after inhibition of dynein function. Mean proplatelet perimeter increases by ∼1.5-fold over the course of 9 hours during static tissue culture. The addition of 1 mM Na₃V0₄ or EHNA (broad-spectrum cytoplasmic dynein inhibitors) at 6 hours inhibits further proplatelet elongation. Comparable inhibition of proplatelet elongation was attained with 100 µM Ciliobrevin D (specific cytoplasmic dynein inhibitor) when added at 0 hours, but not at 6 hours, of culture. Analyses were performed for at least 4 independent samples. Error bars represent 1 standard deviation about the mean. Statistical significance was established using a 1-tailed Student t test for paired samples (**P < .01, *P < .05).