For both MoO42- and WO42- the maximum rate of uptake by the small intestine of the rat (studied in vitro using the everted sac technique) occurs in the lower ileum. Kinetic constants, derived by a least squares procedure, are compared with those previously obtained for SO42- transport. For both V and Ka, SO42- greater than MoO42- greater than WO42-, with only small differences between sacs IV and V. Mutual inhibition of MoO42- and WO42- transport and inhibition of both by SO42- are competitive processes. This is shown by the generally good agreement between Ka values and derived Ki values and by V values in the presence and absence of the inhibiting species. The three ions SO42-, MoO42- and WO42- are probably transferred across the intestine by a common carrier system. Implications for the sulphate-molybdenum interaction in molybdosis are discussed.