The blood clotting time of the chick varies with age, with the vit. K reserve of the chick, and with the vit. K level in the ration, and it tends to reach a maximum at 2 wks. of age. At 3 wks. of age the chick achieves a balance of blood clotting power with respect to the vit. K level in the ration. The reciprocal of the blood clotting time is a simple linear function of the logarithm of the vit. K level in the ration, over a practical range of values. Chicks free from hemorrhage but with greatly prolonged blood clotting times had normal Hb levels. An improved assay procedure for vit. K is suggested.