Human uterine fluid (UF) was found to contain several amino acids at high concentrations. Taurine was predominant; its concentration in UF was up to 400-fold that of plasma during the luteal phase. High UF concentrations of amino acids probably result from active endometrial secretion, which is selective with respect to type of amino acid and is regulated by ovarian steroids. Amino acids, particularly taurine, have an apparent function in osmotic regulation since UF has a lower salt content than serum but the same osmolarity. The high concentration of taurine may also act to sustain the motility of spermatozoa in UF, by protecting them from the adverse influence of high K+ concentration. Knowledge of the amino acid composition of UF in the luteal phase may prove useful in such clinical contexts as in vitro fertilization of ova and embryo replacement.