The effects of thrombopoietic stimulation on megakaryocytopoiesis, platelet production, and platelet viability and function were examined in normal volunteers randomized to receive single bolus subcutaneous injections of 3 microg/kg pegylated recombinant megakaryocyte growth and development factor (PEG-rHuMGDF) or placebo in a 3:1 ratio. PEG-rHuMGDF transiently doubled circulating platelet counts, from 237 +/- 41 x 10(3)/microL to 522 +/- 90 x 10(3)/microL (P <.0001), peaking on day 12. Baseline and day-12 samples showed no differences in responsiveness of platelets to adenosine diphosphate or thrombin receptor agonist peptide (P >.4 in all cases); expression of platelet ligand-induced binding sites or annexin V binding sites (P >.6 in both cases); or density of platelet TPO-receptors (P >.5). Platelet counts normalized by day 28. The life span of autologous (111)In-labeled platelets increased from 205 +/- 18 hours (baseline) to 226 +/- 22 hours (P <.01) on day 8. Platelet life span decreased from 226 +/- 22 hours (day 8) to 178 +/- 53 hours (P <.05) on day 18. The theoretical basis for senescent changes in mean platelet life span was illustrated by biomathematical modeling. Platelet turnover increased from 43.9 +/- 11.9 x 10(3) platelets/microL/d (baseline) to 101 +/- 27.6 x 10(3) platelets/microL/d (P =.0009), and marrow megakaryocyte mass expanded from 37.4 +/- 18.5 fL/kg to 62 +/- 17 x 10(10) fL/kg (P =. 015). Although PEG-rHuMGDF initially increased megakaryocyte volume and ploidy, subsequently ploidy showed a transient reciprocal decrease when the platelet counts exceeded placebo values. In healthy human volunteers PEG-rHuMGDF transiently increases megakaryocytopoiesis 2-fold. Additionally, peripheral platelets expand correspondingly and exhibit normal function and viability during the ensuing 10 days. The induced perturbation in steady state thrombopoiesis resolves by 4 weeks. (Blood. 2000;95:2514-2522)