(1) When the blue dye T-1824 is injected into the bloodstream of dogs, mixing of the dye in the plasma occurs in 2 phases. Phase I, which is completed 4-6 mins. after injn., represents mixing of the dye in the circulating plasma. Phase II, which is completed 30-50 mins. after injn., reflects the diffusion of the dye into non-circulating plasma. (2) Extrapolation of the phase II portion of the mixing curve permits the calculation of circulating plasma vol., while extrapolation of the true disappearance curve (i.e., the time-concn. curve after completion of phase II) provides an index of total plasma vol. Based on this type of calculation, the circulating plasma vol. in normal, unanesthe-tized dogs is approx. 86% of the total plasma vol.