1. The plasma viscosity values of normal commercial broiler and layer strains of chicken were measured in relation to age and sex. 2. In both broilers and layers plasma viscosity values decreased at 2 weeks of age; probably reflecting the period of change from maternal plasma protein to the bird's self-synthesised proteins. 3. There was a progressive rise in plasma viscosity in both strains and sexes with age. The increase was greatest in female layers at 17 weeks of age before lay. 4. There was no overall significant difference in viscosity values between fresh and frozen-stored plasma samples.