By means of neutron activation analysis, the concentrations of zinc, copper, manganese, sodium and potassium were studied in endometrial biopsy specimens taken from six apparently healthy, normally menstruating volunteers. Quadruplicate endometrial biopsy specimens were obtained from each of the subjects during four consecutive cycles in the following phases: a) early proliferative (days 6–10), b) late proliferative (days 11–14), c) early secretory (days 15–18) and d) late secretory (days 22–27). An analysis of variance of the data revealed that the human endometrium in the early proliferative phase is characterized by significantly elevated concentrations of manganese (P < 0.001), sodium (P < 0.01) and potassium (P < 0.001). On the other hand, the late secretory endometrium is characterized by a highly significant rise in its zinc concentration (P < 0.001), accompanied by a highly significantly decreased concentration of sodium (P < 0.001) and potassium (P < 0.001). The copper concentration of the secretory endometria was significantly higher than that of the proliferative endometria (P < 0.001). The significance of the findings was the same, whether values were expressed per g protein, or per g wet tissue. It is suggested that the high concentrations of zinc and copper associated with low levels of manganese, sodium and potassium at the expected time of implantation may be a reflection of changes in endometrial enzyme activities.