The hematologic development of Japanese quail was studied as a function of time through the first 49 hr post-hatching. Packed cell volumes, hemoglobin levels, erythrocyte and leukocyte numbers, as well as differential percentages of lymphocytes, heterophils, basophils, eosinophils, and monocytes were determined. Additionally, the mean corpuscular volumes, mean corpuscular hemoglobins, and mean corpuscular hemoglobin concentrations were calculated. Of particular interest are the increasing mean corpuscular volume and the increasing number of erythrocytes which were observed as the birds aged. Between the 13th and 19th hr after hatching, an almost 2-fold increase in the erythrocyte number was detected with a concomitant decrease in the mean corpuscular volume. These findings indicate that the state of the hematologic system in quail is highly dependent on the stage of post-embryonic maturation.