Methods used to measure the complex dielectric constant of water and blood at frequencies from 1.7 × 10⁹ to 2.4 × 10¹⁰ c/s are described. The results obtained for water at temperatures between 0 and 60° C are given and analysed with relation to the Debye and the Cole-Cole dispersion equations. The possibility that the dispersion in water is characterized by a narrow spectrum of relaxation times is briefly discussed. The complex dielectric constant of whole human blood has been measured at temperatures between 15 and 35° C. The dispersion observed is attributed entirely to water relaxation. It is shown to fit the Debye dispersion equations if the effects of a frequency-independent ionic conductivity are allowed for. Comparison of the results for blood with those for water leads to approximate estimates of the erythrocyte intracellular ionic conductivity and haemoglobin hydration.