Histochemical techniques, including radioisotope histochemistry, have been used to investigate the nature of the surface carbohydrates at the apex of cells of the luminal epithelium of the rat uterus under various hormonal conditions. Binding of ruthenium red was quantitatively similar in ovariectomized rats without further treatment and in those given three daily injections of progesterone. Ruthenium red binding was significantly lower after 3 days treatment with estradiol, and also after 3 days treatment with progesterone with an additional dose of estradiol on day 3, a regime known to produce an epithelium receptive to the implanting blastocyst. Binding of concanavalin A (con A), whether studied by electron microscope histochemistry after incubation of tissue with con A-horseradish peroxidase, or by light microscope autoradiography after incubation with 3H-con A, was not statistically different in any of the four groups of rats. The results with ruthenium red show a reduction in net negative charge of the carbohydrates on the apical cell membrane in conditions permitting implantation: this change is not due to variations in the amounts of the neutral carbohydrates, mannose and glucose, as demonstrated by con A.