The transport of phosphate into jejunal brush border membrane vesicles isolated from 14- to 42-day-old rats was investigated using a rapid filtration technique. The presence of a sodium gradient enhanced phosphate uptake at all ages. Both sodium-dependent phosphate uptake and passive phosphate uptake declined with increasing age. The activity of the sodium/phosphate cotransporter was significantly greater in the 14-day-old suckling animals than in the 42-day-old animals. Experiments with valinomycin in 14- and 42-day-old animals showed that at pH 7.4, phosphate uptake is electroneutral at both ages. These findings demonstrate there are maturational changes in the jejunal transport of phosphate. Suckling animals transport significantly more phosphate than adult animals. The increased uptake in the younger animals is related to an increase in the active uptake of phosphate and an increase in the passive permeability to phosphate.