The purpose of this study was to examine thi hypothesis that in humans the sex of the fetus can be determined by fetal heart rate. Data were collected from 250 male and 250 female births on a number of perinatal and maternal variables that could affect fetal heart rate. The measures included (1) the sex of the child, (2) the fetal heart rate ausculated numerous times from admission to the hospital until the administration of obstetrical drugs, (3) the weight and length of the neonate, (4) the birth order of the child, (5) the maternal heart rate, (6) the weight gain of the mother during pregnancy, (7) the weight and age of the mother. Data from 41 families in which one child of each sex had been born were also compiled. The data did not support the hypothesis; the inclusion of the other variables in the analysis did not improve the correlation. Explanations are suggested for the results and the origin of the hypothesis.