Planimetric studies were carried out on canine blood platelets fixed with formaldehyde. In the normal dog, the mean platelet area was 5.25 sqµ (SEM ± 0.34 sqµ). During the first 3 days following acute experimental thrombocytopenia, platelet area increased, averaging 9.5 sqµ (p < 0.01). The reactive thrombocytosis following splenectomy or nephrectomy was not accompanied by a rise in large platelets. Platelet size decreased to 3.9 sqµ in dogs made thrombocytopenic by intravenous injections of mitomycin C. The morphologic changes induced by the various experimental procedures suggest that: (1) newly formed platelets are larger than platelets of a normal population; (2) some large platelets are possibly released by macromegakaryocytes; (3) other large thrombocytes probably represent fragments of granular megakaryocytes; (4) senescence of platelets in the circulation is associated with decreasing size; and (5) large young platelets are preferentially retained in vivo on glass beads.