Haematological values have been determined in 40 goslings, aged 6 to 42 d, and in 86 chickens, aged 1 to 57 d. 2. The erythrocyte number of goslings remained virtually unchanged with age but there was a decrease in the volume of circulating blood when expressed in terms of body weight. 3. In chickens the number of erythrocytes per unit volume increased with age but the size of the erythrocytes diminished with the result that there was an increase in the red blood cell total surface area per unit volume. 4. The haemoglobin content of chicken blood also rose but the haematocrit value and the mead cell haemoglobin concentration remained constant: mean cell haemoglobin decreased significantly. 5. It seems probable that the blood of goslings has a smaller oxygen carrying capability that that of the chicken and this might be reflected with their slower metabolic rate per body weight unit. There are only a few data available on the changes in haematological values of the domestic fowl after hatching (Lange, 1919; Sandreuter, 1951; Lucas and Denington, 1957; Medway and Kare, 1959) and virtually none for the goose. In this paper various haematological values for both species are presented for the first weeks of post-embryonic life.
