In endocytosis small GTPases Rab5 and Rab7 act like molecular switches and regulate vesicular transport and membrane trafficking. Rab5 represents a marker of early endosomes which is essential for homotypic and heterotypic fusion as well for endosome motility while Rab7 represents a marker of late endosomes. Conversion from Rab5 to Rab7 is important for the course of an endolysosomal path. Purpose of this thesis was to investigate correlation between endosome maturation and proplatelet formation. We used Western blot analysis to examine the level of expression of early (Rab5), late (Rab7) and recycling (Rab11) endosomes in different maturation stages of mouse megakaryocytes and platelets. By immunofluorescence and confocal microscopy, we analyzed the localization of early (Rab5, EEA1) and late (Rab7, LAMP1) endosomes considering the maturation stage of megakaryocytes. In addition, we treated megakaryocytes with inhibitors that cause rapid alkalinization of acidic organelles and block conversion from early to late endosomes to investigate impact on proplatelet formation and megakaryocyte morphology. Lastly, we transfected Dami megakaryocytic cell line with Rab5 and its constitutively active Q79L or dominantly negative N133L mutant to examine differences in phenotype and to determine possible changes in megakaryocyte maturity (expression of marker CD42c and size of transfected cells). Our key results have shown that inhibition of conversion from early to late endosome resulted in reduced proplatelet formation and morphological changes in megakaryocytes. Transfection of mature Dami cells with Rab5 (wt and constitutively active) confirmed a presence of functional early endosomes. Also, constitutively active Rab5 maturation of Dami cells into higher stages in comparison with other genotypes and control. Our results point out interconnection of endocytosis and process of platelet precursors assembly. Future experiments will have a crucial contribution in defining the general mechanism of this processes.