Significant and sustained thrombocytopenia in mice maintained in a polycythemic state with weekly injections of washed packed RBCs prompted the examination of the relationship between the observed thrombocytopenia and the polycythemic state. Platelet production was determined by measurement of 75SeM incorporation into platelets. Platelet survival was studied with 51Cr-labeled platelets. Platelet sequestration was assessed by measuring the trapped platelet 51Cr-bound radioactivity in the spleens of these animals. Blood volume was calculated by measuring the dilution of injected 59Fe-tagged RBCs. These studies failed to reveal decreased platelet production, a shortening of the platelet survival, or significant splenic pooling as factors contributing to the observed thrombocytopenia. The blood volumes in the polycythemic group of animals were 8.9% +/- 0.15 (1 S.D.) body weight vs. 6.4% +/- 0.69 in the normals (p less than 0.001). Contrary to what has been reported for hypoxia-induced polycythemia, our results indicate that transfusion-induced polycythemia does not cause thrombocytopenia by decreasing platelet production. We suggest instead that the observed thrombocytopenia is mainly due to dilution of the circulating platelet mass by the expanded blood volume.