In a series of foetal lambs weighing between 1,100 and 5,228 g, the circulating plasma volume was estimated by the dye dilution method, using Evans Blue, to test the possibility that the plasma volume could be used as an index of foetal weight in chronic studies. The data, analysed by the method of least squares regression, indicate that plasma volume and foetal weight are closely correlated (R-2 equals 0.922) and linearly so in the range of data studied. There was no evidence that the relation differed for singlets and twins. A single equation, Y equals 71.8 plus 10.11 X--where Y is the estimated weight and X the plasma volume, can be used to predict the weight from plasma volume in both. Some results of the application of the method in chronic studies are presented.