The deuterated retinol dilution technique is an indirect method for quantitatively estimating total body stores of vitamin A by using the postequilibration plasma isotopic ratio [2H4]retinol:retinol and the prediction model described by Furr et al (Am J Clin Nutr 1989;49:713-6). Limited data are available on the time required for an oral dose of labeled vitamin A to mix with vitamin A body stores in human subjects. This article describes the plasma retinol kinetics of an oral dose of [2H4] retinyl acetate in 4 healthy adults (2 men and 2 women) and 1 healthy female child in the United States and in 4 Bangladeshi women. After an oral dose of [2H4]retinyl acetate was administered, plasma samples were collected at 6, 12, and 24 h postdose during the first day and at 15 time points during the subsequent 90-d period for measurement of plasma [2H4]retinol:retinol. The mean respective plasma isotopic ratios on day 20 for US and Bangladeshi subjects (0.02 +/- 0.02 and 0.17 +/- 0.12, P = 0.03) and estimated total body vitamin A reserves (1.03 +/- 0.45 and 0.10 +/- 0.11 mmol, P = 0.003) were significantly different. The fraction of dose in plasma was plotted against time, and biexponential equations were fit to the kinetic data by using the time points from 24 h through day 90. The mean equilibration time (time required for the fraction of dose in plasma to reach a plateau) for all subjects was 16.6 +/- 3.8 d (11-23 d). There was no difference in estimated equilibration time between the group of US and Bangladeshi adult subjects (17.5 +/- 4.4 and 16.3 +/- 3.9 d, respectively, P = 0.69). Thus, the size of hepatic vitamin A reserves does not appear to affect equilibration time within the range of values observed.